v5.1.2

fix(readme): adjust ASCII architecture diagram alignment in README
v5.1.1
2026-02-11 10:20:19 +00:00 · 2026-02-11 10:20:19 +00:00 · 2026-02-11 10:16:30 +00:00 · 2026-02-11 10:16:30 +00:00 · 2026-02-11 10:11:43 +00:00 · 2026-02-11 10:11:43 +00:00
30 changed files with 1878 additions and 170 deletions
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,32 @@
 # Changelog

+## 2026-02-11 - 5.1.2 - fix(readme)
+adjust ASCII architecture diagram alignment in README
+
+- Whitespace and alignment tweaks to the ASCII architecture diagram in readme.md
+- No code or behavior changes; documentation-only edit
+
+## 2026-02-11 - 5.1.1 - fix(release)
+no changes
+
+- No files changed in this commit.
+- Current package version remains 5.1.0 (from package.json).
+
+## 2026-02-11 - 5.1.0 - feat(mailer-smtp)
+add SCRAM-SHA-256 auth, Ed25519 DKIM, opportunistic TLS, SNI cert selection, pipelining and delivery/bridge improvements
+
+- Add server-side SCRAM-SHA-256 implementation in Rust (scram.rs) and wire up SCRAM credential request/response between Rust and TypeScript bridge (ScramCredentialRequest / scramCredentialResult).
+- Support SCRAM-SHA-256 auth mechanism in SMTP command parsing and advertise AUTH PLAIN LOGIN SCRAM-SHA-256 capability.
+- Add opportunistic TLS mode for MTA-to-MTA delivery: configurable tls_opportunistic flag, an OpportunisticVerifier that skips cert verification per RFC 7435, and plumbing into connect/upgrade TLS paths.
+- Add pipelined envelope support for MAIL FROM + multiple RCPT TO (send_pipelined_envelope) and use pipelining when server advertises PIPELINING to improve outbound performance.
+- Add Ed25519 DKIM signing support and auto-dispatch: sign_dkim_ed25519, sign_dkim_auto, dkim_dns_record_value_typed, and TS changes to detect key type and call the auto signing API.
+- Expose additional per-domain TLS certs (additionalTlsCerts) and implement SNI-based certificate resolver on the server to select certs by hostname; parsing helpers and fallback default cert handling included.
+- Install ring crypto provider early in mailer-bin main for rustls operations and add related rust dependencies (sha2, hmac, pbkdf2) and workspace entries.
+- TypeScript delivery and server bridge changes: group recipients by domain, MX resolution fallback to A record, MTA delivery loop over MX hosts, DKIM options propagation, TLS opportunistic option passed to outbound client, SCRAM credential computation in TS using PBKDF2/HMAC/SHA256 and sending results back to Rust.
+- Add new tests and utilities: IPv6 DNSBL support and tests, SCRAM unit tests, DKIM Ed25519 tests, node-level MTA delivery integration test, and various test updates.
+- Public API additions on the Rust <-> TS bridge: signDkim accepts keyType, new scram credential result command, onScramCredentialRequest/onScramCredentialResult helpers and sendScramCredentialResult.
+- Various refactors and safety/feature improvements across mailer-core/smtp/security: envelope handling, stream buffering detection, and error handling for auth flows.
+
 ## 2026-02-11 - 5.0.0 - BREAKING CHANGE(mail)
 remove DMARC and DKIM verifier implementations and MTA error classes; introduce DkimManager and EmailActionExecutor; simplify SPF verifier and update routing exports and tests

--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
  "name": "@push.rocks/smartmta",
-  "version": "5.0.0",
+  "version": "5.1.2",
  "description": "A high-performance, enterprise-grade Mail Transfer Agent (MTA) built from scratch in TypeScript with Rust acceleration.",
  "keywords": [
    "mta",
--- a/readme.md
+++ b/readme.md
@@ -43,28 +43,28 @@ After installation, run `pnpm build` to compile the Rust binary (`mailer-bin`).

 ```
 ┌──────────────────────────────────────────────────────────────┐
-│                    UnifiedEmailServer                         │
-│     (orchestrates all components, emits events)               │
+│                    UnifiedEmailServer                        │
+│     (orchestrates all components, emits events)              │
 ├───────────┬───────────┬──────────────┬───────────────────────┤
 │   Email   │  Security │  Delivery    │  Configuration        │
 │   Router  │  Stack    │  System      │                       │
-│  ┌──────┐ │ ┌───────┐ │ ┌──────────┐ │  ┌────────────────┐  │
-│  │Match │ │ │ DKIM  │ │ │ Queue    │ │  │ DomainRegistry │  │
-│  │Route │ │ │ SPF   │ │ │ Rate Lim │ │  │ DnsManager     │  │
-│  │ Act  │ │ │ DMARC │ │ │ Retry    │ │  │ DKIMCreator    │  │
-│  └──────┘ │ │ IPRep │ │ └──────────┘ │  │ Templates      │  │
-│           │ │ Scan  │ │              │  └────────────────┘  │
+│  ┌──────┐ │ ┌───────┐ │ ┌──────────┐ │  ┌────────────────┐   │
+│  │Match │ │ │ DKIM  │ │ │ Queue    │ │  │ DomainRegistry │   │
+│  │Route │ │ │ SPF   │ │ │ Rate Lim │ │  │ DnsManager     │   │
+│  │ Act  │ │ │ DMARC │ │ │ Retry    │ │  │ DKIMCreator    │   │
+│  └──────┘ │ │ IPRep │ │ └──────────┘ │  │ Templates      │   │
+│           │ │ Scan  │ │              │  └────────────────┘   │
 │           │ └───────┘ │              │                       │
 ├───────────┴───────────┴──────────────┴───────────────────────┤
-│              Rust Security Bridge (smartrust IPC)             │
+│              Rust Security Bridge (smartrust IPC)            │
 ├──────────────────────────────────────────────────────────────┤
-│                   Rust Acceleration Layer                      │
-│  ┌──────────────┐  ┌───────────────┐  ┌──────────────────┐  │
-│  │ mailer-smtp  │  │mailer-security│  │   mailer-core    │  │
-│  │ SMTP Server  │  │DKIM/SPF/DMARC │  │ Types/Validation │  │
-│  │ SMTP Client  │  │IP Rep/Content │  │ MIME/Bounce      │  │
-│  │ TLS/AUTH     │  │   Scanning    │  │   Detection      │  │
-│  └──────────────┘  └───────────────┘  └──────────────────┘  │
+│                   Rust Acceleration Layer                    │
+│  ┌──────────────┐  ┌───────────────┐  ┌──────────────────┐   │
+│  │ mailer-smtp  │  │mailer-security│  │   mailer-core    │   │
+│  │ SMTP Server  │  │DKIM/SPF/DMARC │  │ Types/Validation │   │
+│  │ SMTP Client  │  │IP Rep/Content │  │ MIME/Bounce      │   │
+│  │ TLS/AUTH     │  │   Scanning    │  │   Detection      │   │
+│  └──────────────┘  └───────────────┘  └──────────────────┘   │
 └──────────────────────────────────────────────────────────────┘
 ```

--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -982,6 +982,7 @@ dependencies = [
 "mailer-core",
 "mailer-security",
 "mailer-smtp",
+ "rustls",
 "serde",
 "serde_json",
 "tokio",
@@ -1025,15 +1026,18 @@ dependencies = [
 "base64",
 "dashmap",
 "hickory-resolver 0.25.2",
+ "hmac",
 "mailer-core",
 "mailer-security",
 "mailparse",
+ "pbkdf2",
 "regex",
 "rustls",
 "rustls-pemfile",
 "rustls-pki-types",
 "serde",
 "serde_json",
+ "sha2",
 "thiserror",
 "tokio",
 "tokio-rustls",
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -29,3 +29,6 @@ uuid = { version = "1", features = ["v4"] }
 rustls-pki-types = "1"
 psl = "2"
 clap = { version = "4", features = ["derive"] }
+sha2 = "0.10"
+hmac = "0.12"
+pbkdf2 = { version = "0.12", default-features = false }
--- a/rust/crates/mailer-bin/Cargo.toml
+++ b/rust/crates/mailer-bin/Cargo.toml
@@ -21,3 +21,4 @@ hickory-resolver.workspace = true
 dashmap.workspace = true
 base64.workspace = true
 uuid.workspace = true
+rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }
--- a/rust/crates/mailer-bin/src/main.rs
+++ b/rust/crates/mailer-bin/src/main.rs
@@ -14,7 +14,7 @@ use std::sync::Arc;
 use tokio::sync::oneshot;

 use mailer_smtp::connection::{
-    AuthResult, CallbackRegistry, ConnectionEvent, EmailProcessingResult,
+    AuthResult, CallbackRegistry, ConnectionEvent, EmailProcessingResult, ScramCredentialResult,
 };

 /// mailer-bin: Rust-powered email security tools
@@ -114,10 +114,11 @@ struct IpcEvent {

 // --- Pending callbacks for correlation-ID based reverse calls ---

-/// Stores oneshot senders for pending email processing and auth callbacks.
+/// Stores oneshot senders for pending email processing, auth, and SCRAM callbacks.
 struct PendingCallbacks {
    email: DashMap<String, oneshot::Sender<EmailProcessingResult>>,
    auth: DashMap<String, oneshot::Sender<AuthResult>>,
+    scram: DashMap<String, oneshot::Sender<ScramCredentialResult>>,
 }

 impl PendingCallbacks {
@@ -125,6 +126,7 @@ impl PendingCallbacks {
        Self {
            email: DashMap::new(),
            auth: DashMap::new(),
+            scram: DashMap::new(),
        }
    }
 }
@@ -147,9 +149,22 @@ impl CallbackRegistry for PendingCallbacks {
        self.auth.insert(correlation_id.to_string(), tx);
        rx
    }
+
+    fn register_scram_callback(
+        &self,
+        correlation_id: &str,
+    ) -> oneshot::Receiver<ScramCredentialResult> {
+        let (tx, rx) = oneshot::channel();
+        self.scram.insert(correlation_id.to_string(), tx);
+        rx
+    }
 }

 fn main() {
+    // Install the ring CryptoProvider for rustls TLS operations (STARTTLS, implicit TLS).
+    // This must happen before any TLS connection is attempted.
+    let _ = rustls::crypto::ring::default_provider().install_default();
+
    let cli = Cli::parse();

    if cli.management {
@@ -494,6 +509,22 @@ fn handle_smtp_event(event: ConnectionEvent) {
                }),
            );
        }
+        ConnectionEvent::ScramCredentialRequest {
+            correlation_id,
+            session_id,
+            username,
+            remote_addr,
+        } => {
+            emit_event(
+                "scramCredentialRequest",
+                serde_json::json!({
+                    "correlationId": correlation_id,
+                    "sessionId": session_id,
+                    "username": username,
+                    "remoteAddr": remote_addr,
+                }),
+            );
+        }
    }
 }

@@ -642,8 +673,13 @@ async fn handle_ipc_request(req: &IpcRequest, state: &mut ManagementState) -> Ip
                .get("privateKey")
                .and_then(|v| v.as_str())
                .unwrap_or("");
+            let key_type = req
+                .params
+                .get("keyType")
+                .and_then(|v| v.as_str())
+                .unwrap_or("rsa");

-            match mailer_security::sign_dkim(raw_message.as_bytes(), domain, selector, private_key) {
+            match mailer_security::sign_dkim_auto(raw_message.as_bytes(), domain, selector, private_key, key_type) {
                Ok(header) => IpcResponse {
                    id: req.id.clone(),
                    success: true,
@@ -825,6 +861,10 @@ async fn handle_ipc_request(req: &IpcRequest, state: &mut ManagementState) -> Ip
            handle_auth_result(req, state)
        }

+        "scramCredentialResult" => {
+            handle_scram_credential_result(req, state)
+        }
+
        "configureRateLimits" => {
            // Rate limit configuration is set at startSmtpServer time.
            // This command allows runtime updates, but for now we acknowledge it.
@@ -1010,6 +1050,56 @@ fn handle_auth_result(req: &IpcRequest, state: &ManagementState) -> IpcResponse
    }
 }

+/// Handle scramCredentialResult IPC command — resolves a pending SCRAM credential callback.
+fn handle_scram_credential_result(req: &IpcRequest, state: &ManagementState) -> IpcResponse {
+    use base64::Engine;
+    use base64::engine::general_purpose::STANDARD as BASE64;
+
+    let correlation_id = req
+        .params
+        .get("correlationId")
+        .and_then(|v| v.as_str())
+        .unwrap_or("");
+
+    let found = req.params.get("found").and_then(|v| v.as_bool()).unwrap_or(false);
+
+    let result = ScramCredentialResult {
+        found,
+        salt: req.params.get("salt")
+            .and_then(|v| v.as_str())
+            .and_then(|s| BASE64.decode(s.as_bytes()).ok()),
+        iterations: req.params.get("iterations")
+            .and_then(|v| v.as_u64())
+            .map(|n| n as u32),
+        stored_key: req.params.get("storedKey")
+            .and_then(|v| v.as_str())
+            .and_then(|s| BASE64.decode(s.as_bytes()).ok()),
+        server_key: req.params.get("serverKey")
+            .and_then(|v| v.as_str())
+            .and_then(|s| BASE64.decode(s.as_bytes()).ok()),
+    };
+
+    if let Some((_, tx)) = state.callbacks.scram.remove(correlation_id) {
+        let _ = tx.send(result);
+        IpcResponse {
+            id: req.id.clone(),
+            success: true,
+            result: Some(serde_json::json!({"resolved": true})),
+            error: None,
+        }
+    } else {
+        IpcResponse {
+            id: req.id.clone(),
+            success: false,
+            result: None,
+            error: Some(format!(
+                "No pending SCRAM credential callback for correlationId: {}",
+                correlation_id
+            )),
+        }
+    }
+}
+
 /// Parse SmtpServerConfig from IPC params JSON.
 fn parse_smtp_config(
    params: &serde_json::Value,
@@ -1075,6 +1165,27 @@ fn parse_smtp_config(
        config.processing_timeout_secs = timeout;
    }

+    // Parse additional TLS certs for SNI
+    if let Some(certs_arr) = params.get("additionalTlsCerts").and_then(|v| v.as_array()) {
+        for cert_val in certs_arr {
+            if let (Some(domains_arr), Some(cert_pem), Some(key_pem)) = (
+                cert_val.get("domains").and_then(|v| v.as_array()),
+                cert_val.get("certPem").and_then(|v| v.as_str()),
+                cert_val.get("keyPem").and_then(|v| v.as_str()),
+            ) {
+                let domains: Vec<String> = domains_arr
+                    .iter()
+                    .filter_map(|v| v.as_str().map(String::from))
+                    .collect();
+                config.additional_tls_certs.push(mailer_smtp::config::TlsDomainCert {
+                    domains,
+                    cert_pem: cert_pem.to_string(),
+                    key_pem: key_pem.to_string(),
+                });
+            }
+        }
+    }
+
    Ok(config)
 }

@@ -1187,11 +1298,12 @@ async fn handle_send_email(req: &IpcRequest, state: &ManagementState) -> IpcResp
    // Optional DKIM signing
    if let Some(dkim_val) = req.params.get("dkim") {
        if let Ok(dkim_config) = serde_json::from_value::<mailer_smtp::client::DkimSignConfig>(dkim_val.clone()) {
-            match mailer_security::sign_dkim(
+            match mailer_security::sign_dkim_auto(
                &raw_message,
                &dkim_config.domain,
                &dkim_config.selector,
                &dkim_config.private_key,
+                &dkim_config.key_type,
            ) {
                Ok(header) => {
                    // Prepend DKIM header to the message
--- a/rust/crates/mailer-security/src/dkim.rs
+++ b/rust/crates/mailer-security/src/dkim.rs
@@ -1,4 +1,4 @@
-use mail_auth::common::crypto::{RsaKey, Sha256};
+use mail_auth::common::crypto::{Ed25519Key, RsaKey, Sha256};
 use mail_auth::common::headers::HeaderWriter;
 use mail_auth::dkim::{Canonicalization, DkimSigner};
 use mail_auth::{AuthenticatedMessage, DkimOutput, DkimResult, MessageAuthenticator};
@@ -118,9 +118,62 @@ pub fn sign_dkim(
    Ok(signature.to_header())
 }

+/// Sign a raw email message with DKIM using Ed25519-SHA256 (RFC 8463).
+///
+/// * `raw_message` - The raw RFC 5322 message bytes
+/// * `domain` - The signing domain (d= tag)
+/// * `selector` - The DKIM selector (s= tag)
+/// * `private_key_pkcs8_der` - Ed25519 private key in PKCS#8 DER format
+///
+/// Returns the DKIM-Signature header string to prepend to the message.
+pub fn sign_dkim_ed25519(
+    raw_message: &[u8],
+    domain: &str,
+    selector: &str,
+    private_key_pkcs8_der: &[u8],
+) -> Result<String> {
+    let ed_key = Ed25519Key::from_pkcs8_maybe_unchecked_der(private_key_pkcs8_der)
+        .map_err(|e| SecurityError::Key(format!("Failed to load Ed25519 key: {}", e)))?;
+
+    let signature = DkimSigner::from_key(ed_key)
+        .domain(domain)
+        .selector(selector)
+        .headers(["From", "To", "Subject", "Date", "Message-ID", "MIME-Version", "Content-Type"])
+        .header_canonicalization(Canonicalization::Relaxed)
+        .body_canonicalization(Canonicalization::Relaxed)
+        .sign(raw_message)
+        .map_err(|e| SecurityError::Dkim(format!("Ed25519 DKIM signing failed: {}", e)))?;
+
+    Ok(signature.to_header())
+}
+
+/// Sign a raw email message with DKIM, auto-selecting RSA or Ed25519 based on `key_type`.
+///
+/// * `key_type` - `"rsa"` (default) or `"ed25519"`
+/// * For RSA: `private_key_pem` is a PEM-encoded RSA key
+/// * For Ed25519: `private_key_pem` is a PEM-encoded PKCS#8 Ed25519 key
+pub fn sign_dkim_auto(
+    raw_message: &[u8],
+    domain: &str,
+    selector: &str,
+    private_key_pem: &str,
+    key_type: &str,
+) -> Result<String> {
+    match key_type.to_lowercase().as_str() {
+        "ed25519" => {
+            // Parse PEM to DER for Ed25519
+            let der = rustls_pki_types::PrivatePkcs8KeyDer::from_pem_slice(private_key_pem.as_bytes())
+                .map_err(|e| SecurityError::Key(format!("Failed to parse Ed25519 PEM: {}", e)))?;
+            sign_dkim_ed25519(raw_message, domain, selector, der.secret_pkcs8_der())
+        }
+        _ => sign_dkim(raw_message, domain, selector, private_key_pem),
+    }
+}
+
 /// Generate a DKIM DNS TXT record value for a given public key.
 ///
 /// Returns the value for a TXT record at `{selector}._domainkey.{domain}`.
+/// `key_type` should be `"rsa"` or `"ed25519"`.
 pub fn dkim_dns_record_value(public_key_pem: &str) -> String {
    // Extract the base64 content from PEM
    let key_b64: String = public_key_pem
@@ -132,6 +185,24 @@ pub fn dkim_dns_record_value(public_key_pem: &str) -> String {
    format!("v=DKIM1; h=sha256; k=rsa; p={}", key_b64)
 }

+/// Generate a DKIM DNS TXT record value with explicit key type.
+///
+/// * `key_type` - `"rsa"` or `"ed25519"`
+pub fn dkim_dns_record_value_typed(public_key_pem: &str, key_type: &str) -> String {
+    let key_b64: String = public_key_pem
+        .lines()
+        .filter(|line| !line.starts_with("-----"))
+        .collect::<Vec<_>>()
+        .join("");
+
+    let k = match key_type.to_lowercase().as_str() {
+        "ed25519" => "ed25519",
+        _ => "rsa",
+    };
+
+    format!("v=DKIM1; h=sha256; k={}; p={}", k, key_b64)
+}
+
 #[cfg(test)]
 mod tests {
    use super::*;
@@ -149,4 +220,42 @@ mod tests {
        let result = sign_dkim(b"From: test@example.com\r\n\r\nBody", "example.com", "mta", "not a key");
        assert!(result.is_err());
    }
+
+    #[test]
+    fn test_sign_dkim_ed25519() {
+        // Generate an Ed25519 key pair using mail-auth
+        let pkcs8_der = Ed25519Key::generate_pkcs8().expect("generate ed25519 key");
+        let ed_key = Ed25519Key::from_pkcs8_der(&pkcs8_der).expect("parse ed25519 key");
+        let _pub_key = ed_key.public_key();
+
+        let msg = b"From: test@example.com\r\nTo: rcpt@example.com\r\nSubject: Test\r\n\r\nBody";
+        let result = sign_dkim_ed25519(msg, "example.com", "ed25519sel", &pkcs8_der);
+        assert!(result.is_ok());
+        let header = result.unwrap();
+        assert!(header.contains("a=ed25519-sha256"));
+        assert!(header.contains("d=example.com"));
+        assert!(header.contains("s=ed25519sel"));
+    }
+
+    #[test]
+    fn test_sign_dkim_auto_dispatches() {
+        // RSA with invalid key should error
+        let msg = b"From: test@example.com\r\n\r\nBody";
+        let result = sign_dkim_auto(msg, "example.com", "mta", "not a key", "rsa");
+        assert!(result.is_err());
+
+        // Ed25519 with invalid PEM should error
+        let result = sign_dkim_auto(msg, "example.com", "mta", "not a key", "ed25519");
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_dkim_dns_record_value_typed() {
+        let pem = "-----BEGIN PUBLIC KEY-----\nMIIBIjANBg==\n-----END PUBLIC KEY-----";
+        let rsa_record = dkim_dns_record_value_typed(pem, "rsa");
+        assert!(rsa_record.contains("k=rsa"));
+
+        let ed_record = dkim_dns_record_value_typed(pem, "ed25519");
+        assert!(ed_record.contains("k=ed25519"));
+    }
 }
--- a/rust/crates/mailer-security/src/ip_reputation.rs
+++ b/rust/crates/mailer-security/src/ip_reputation.rs
@@ -1,6 +1,6 @@
 use hickory_resolver::TokioResolver;
 use serde::{Deserialize, Serialize};
-use std::net::{IpAddr, Ipv4Addr};
+use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};

 use crate::error::Result;

@@ -83,7 +83,7 @@ pub fn risk_level(score: u8) -> RiskLevel {

 /// Check an IP against DNSBL servers.
 ///
-/// * `ip` - The IP address to check (must be IPv4)
+/// * `ip` - The IP address to check (IPv4 or IPv6)
 /// * `dnsbl_servers` - DNSBL servers to query (use `DEFAULT_DNSBL_SERVERS` for defaults)
 /// * `resolver` - DNS resolver to use
 pub async fn check_dnsbl(
@@ -91,20 +91,10 @@ pub async fn check_dnsbl(
    dnsbl_servers: &[&str],
    resolver: &TokioResolver,
 ) -> Result<DnsblResult> {
-    let ipv4 = match ip {
-        IpAddr::V4(v4) => v4,
-        IpAddr::V6(_) => {
-            // IPv6 DNSBL is less common; return clean result
-            return Ok(DnsblResult {
-                ip: ip.to_string(),
-                listed_count: 0,
-                listed_on: Vec::new(),
-                total_checked: 0,
-            });
-        }
+    let reversed = match ip {
+        IpAddr::V4(v4) => reverse_ipv4(v4),
+        IpAddr::V6(v6) => reverse_ipv6(v6),
    };
-
-    let reversed = reverse_ipv4(ipv4);
    let total = dnsbl_servers.len();

    // Query all DNSBL servers in parallel
@@ -178,6 +168,21 @@ fn reverse_ipv4(ip: Ipv4Addr) -> String {
    format!("{}.{}.{}.{}", octets[3], octets[2], octets[1], octets[0])
 }

+/// Reverse IPv6 address to nibble format for DNSBL queries.
+///
+/// Expands to full 32-nibble hex, reverses, and dot-separates each nibble.
+/// E.g. `2001:db8::1` -> `1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2`
+fn reverse_ipv6(ip: Ipv6Addr) -> String {
+    let segments = ip.segments();
+    let full_hex: String = segments.iter().map(|s| format!("{:04x}", s)).collect();
+    full_hex
+        .chars()
+        .rev()
+        .map(|c| c.to_string())
+        .collect::<Vec<_>>()
+        .join(".")
+}
+
 /// Heuristic IP type classification based on well-known prefix ranges.
 /// Same heuristics as the TypeScript IPReputationChecker.
 fn classify_ip(ip: IpAddr) -> IpType {
@@ -272,6 +277,38 @@ mod tests {
        assert!(!is_valid_ipv4("not-an-ip"));
    }

+    #[test]
+    fn test_reverse_ipv6() {
+        let ip: Ipv6Addr = "2001:0db8:0000:0000:0000:0000:0000:0001".parse().unwrap();
+        assert_eq!(
+            reverse_ipv6(ip),
+            "1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2"
+        );
+    }
+
+    #[test]
+    fn test_reverse_ipv6_loopback() {
+        let ip: Ipv6Addr = "::1".parse().unwrap();
+        assert_eq!(
+            reverse_ipv6(ip),
+            "1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_check_dnsbl_ipv6_runs() {
+        // Verify IPv6 actually goes through DNSBL queries (not skipped)
+        let resolver = hickory_resolver::TokioResolver::builder_tokio()
+            .map(|b| b.build())
+            .unwrap();
+        let ip: IpAddr = "::1".parse().unwrap();
+        let result = check_dnsbl(ip, DEFAULT_DNSBL_SERVERS, &resolver).await.unwrap();
+        // Loopback should not be listed on any DNSBL
+        assert_eq!(result.listed_count, 0);
+        // But total_checked should be > 0 — proving IPv6 was actually queried
+        assert_eq!(result.total_checked, DEFAULT_DNSBL_SERVERS.len());
+    }
+
    #[test]
    fn test_default_dnsbl_servers() {
        assert_eq!(DEFAULT_DNSBL_SERVERS.len(), 10);
--- a/rust/crates/mailer-security/src/lib.rs
+++ b/rust/crates/mailer-security/src/lib.rs
@@ -9,7 +9,7 @@ pub mod spf;
 pub mod verify;

 // Re-exports for convenience
-pub use dkim::{dkim_dns_record_value, dkim_outputs_to_results, sign_dkim, verify_dkim, DkimVerificationResult};
+pub use dkim::{dkim_dns_record_value, dkim_dns_record_value_typed, dkim_outputs_to_results, sign_dkim, sign_dkim_auto, sign_dkim_ed25519, verify_dkim, DkimVerificationResult};
 pub use dmarc::{check_dmarc, DmarcPolicy, DmarcResult};
 pub use verify::{verify_email_security, EmailSecurityResult};
 pub use error::{Result, SecurityError};
--- a/rust/crates/mailer-smtp/Cargo.toml
+++ b/rust/crates/mailer-smtp/Cargo.toml
@@ -23,3 +23,6 @@ rustls = { version = "0.23", default-features = false, features = ["ring", "logg
 rustls-pemfile = "2"
 mailparse.workspace = true
 webpki-roots = "0.26"
+sha2.workspace = true
+hmac.workspace = true
+pbkdf2.workspace = true
--- a/rust/crates/mailer-smtp/src/client/config.rs
+++ b/rust/crates/mailer-smtp/src/client/config.rs
@@ -37,6 +37,12 @@ pub struct SmtpClientConfig {
    /// Maximum connections per pool. Default: 10.
    #[serde(default = "default_max_pool_connections")]
    pub max_pool_connections: usize,
+
+    /// Accept invalid TLS certificates (expired, self-signed, wrong hostname).
+    /// Standard for MTA-to-MTA opportunistic TLS per RFC 7435.
+    /// Default: false.
+    #[serde(default)]
+    pub tls_opportunistic: bool,
 }

 /// Authentication configuration.
@@ -60,8 +66,15 @@ pub struct DkimSignConfig {
    pub domain: String,
    /// DKIM selector (e.g. "default" or "mta").
    pub selector: String,
-    /// PEM-encoded RSA private key.
+    /// PEM-encoded private key (RSA or Ed25519 PKCS#8).
    pub private_key: String,
+    /// Key type: "rsa" (default) or "ed25519".
+    #[serde(default = "default_key_type")]
+    pub key_type: String,
+}
+
+fn default_key_type() -> String {
+    "rsa".to_string()
 }

 impl SmtpClientConfig {
--- a/rust/crates/mailer-smtp/src/client/connection.rs
+++ b/rust/crates/mailer-smtp/src/client/connection.rs
@@ -117,6 +117,7 @@ pub async fn connect_tls(
    host: &str,
    port: u16,
    timeout_secs: u64,
+    tls_opportunistic: bool,
 ) -> Result<ClientSmtpStream, SmtpClientError> {
    debug!("Connecting to {}:{} (implicit TLS)", host, port);
    let addr = format!("{host}:{port}");
@@ -130,7 +131,7 @@ pub async fn connect_tls(
            message: format!("Failed to connect to {addr}: {e}"),
        })?;

-    let tls_stream = perform_tls_handshake(tcp_stream, host).await?;
+    let tls_stream = perform_tls_handshake(tcp_stream, host, tls_opportunistic).await?;
    Ok(ClientSmtpStream::Tls(BufReader::new(tls_stream)))
 }

@@ -138,24 +139,77 @@ pub async fn connect_tls(
 pub async fn upgrade_to_tls(
    stream: ClientSmtpStream,
    hostname: &str,
+    tls_opportunistic: bool,
 ) -> Result<ClientSmtpStream, SmtpClientError> {
    debug!("Upgrading connection to TLS (STARTTLS) for {}", hostname);
    let tcp_stream = stream.into_tcp_stream()?;
-    let tls_stream = perform_tls_handshake(tcp_stream, hostname).await?;
+    let tls_stream = perform_tls_handshake(tcp_stream, hostname, tls_opportunistic).await?;
    Ok(ClientSmtpStream::Tls(BufReader::new(tls_stream)))
 }

+/// A TLS certificate verifier that accepts any certificate.
+/// Used for MTA-to-MTA opportunistic TLS per RFC 7435.
+#[derive(Debug)]
+struct OpportunisticVerifier;
+
+impl rustls::client::danger::ServerCertVerifier for OpportunisticVerifier {
+    fn verify_server_cert(
+        &self,
+        _end_entity: &rustls_pki_types::CertificateDer<'_>,
+        _intermediates: &[rustls_pki_types::CertificateDer<'_>],
+        _server_name: &rustls_pki_types::ServerName<'_>,
+        _ocsp_response: &[u8],
+        _now: rustls_pki_types::UnixTime,
+    ) -> Result<rustls::client::danger::ServerCertVerified, rustls::Error> {
+        Ok(rustls::client::danger::ServerCertVerified::assertion())
+    }
+
+    fn verify_tls12_signature(
+        &self,
+        _message: &[u8],
+        _cert: &rustls_pki_types::CertificateDer<'_>,
+        _dss: &rustls::DigitallySignedStruct,
+    ) -> Result<rustls::client::danger::HandshakeSignatureValid, rustls::Error> {
+        Ok(rustls::client::danger::HandshakeSignatureValid::assertion())
+    }
+
+    fn verify_tls13_signature(
+        &self,
+        _message: &[u8],
+        _cert: &rustls_pki_types::CertificateDer<'_>,
+        _dss: &rustls::DigitallySignedStruct,
+    ) -> Result<rustls::client::danger::HandshakeSignatureValid, rustls::Error> {
+        Ok(rustls::client::danger::HandshakeSignatureValid::assertion())
+    }
+
+    fn supported_verify_schemes(&self) -> Vec<rustls::SignatureScheme> {
+        rustls::crypto::ring::default_provider()
+            .signature_verification_algorithms
+            .supported_schemes()
+    }
+}
+
 /// Perform the TLS handshake on a TCP stream using webpki-roots.
+/// When `tls_opportunistic` is true, certificate verification is skipped
+/// (standard for MTA-to-MTA delivery per RFC 7435).
 async fn perform_tls_handshake(
    tcp_stream: TcpStream,
    hostname: &str,
+    tls_opportunistic: bool,
 ) -> Result<TlsStream<TcpStream>, SmtpClientError> {
-    let mut root_store = rustls::RootCertStore::empty();
-    root_store.extend(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());
-
-    let tls_config = rustls::ClientConfig::builder()
-        .with_root_certificates(root_store)
-        .with_no_client_auth();
+    let tls_config = if tls_opportunistic {
+        debug!("Using opportunistic TLS (no cert verification) for {}", hostname);
+        rustls::ClientConfig::builder()
+            .dangerous()
+            .with_custom_certificate_verifier(Arc::new(OpportunisticVerifier))
+            .with_no_client_auth()
+    } else {
+        let mut root_store = rustls::RootCertStore::empty();
+        root_store.extend(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());
+        rustls::ClientConfig::builder()
+            .with_root_certificates(root_store)
+            .with_no_client_auth()
+    };

    let connector = tokio_rustls::TlsConnector::from(Arc::new(tls_config));
    let server_name = rustls_pki_types::ServerName::try_from(hostname.to_string()).map_err(|e| {
@@ -190,7 +244,7 @@ mod tests {

    #[tokio::test]
    async fn test_connect_tls_refused() {
-        let result = connect_tls("127.0.0.1", 19998, 2).await;
+        let result = connect_tls("127.0.0.1", 19998, 2, false).await;
        assert!(result.is_err());
    }

--- a/rust/crates/mailer-smtp/src/client/pool.rs
+++ b/rust/crates/mailer-smtp/src/client/pool.rs
@@ -116,6 +116,7 @@ impl ConnectionPool {
                &self.config.host,
                self.config.port,
                self.config.connection_timeout_secs,
+                self.config.tls_opportunistic,
            )
            .await?
        } else {
@@ -139,7 +140,7 @@ impl ConnectionPool {
        if !self.config.secure && capabilities.starttls {
            protocol::send_starttls(&mut stream, self.config.socket_timeout_secs).await?;
            stream =
-                super::connection::upgrade_to_tls(stream, &self.config.host).await?;
+                super::connection::upgrade_to_tls(stream, &self.config.host, self.config.tls_opportunistic).await?;

            // Re-EHLO after STARTTLS — use updated capabilities for auth
            capabilities = protocol::send_ehlo(
@@ -244,9 +245,10 @@ impl SmtpClientManager {
            protocol::send_rset(&mut conn.stream, config.socket_timeout_secs).await?;
        }

-        // Perform the SMTP transaction
+        // Perform the SMTP transaction (use pipelining if server supports it)
+        let pipelining = conn.capabilities.pipelining;
        let result =
-            Self::perform_send(&mut conn.stream, sender, recipients, message, config).await;
+            Self::perform_send(&mut conn.stream, sender, recipients, message, config, pipelining).await;

        // Re-acquire the pool lock and release the connection
        let mut pool = pool_arc.lock().await;
@@ -268,30 +270,39 @@ impl SmtpClientManager {
        recipients: &[String],
        message: &[u8],
        config: &SmtpClientConfig,
+        pipelining: bool,
    ) -> Result<SmtpSendResult, SmtpClientError> {
        let timeout_secs = config.socket_timeout_secs;

-        // MAIL FROM
-        protocol::send_mail_from(stream, sender, timeout_secs).await?;
+        let (accepted, rejected) = if pipelining {
+            // Use pipelined envelope: MAIL FROM + all RCPT TO in one batch
+            let (_mail_ok, acc, rej) = protocol::send_pipelined_envelope(
+                stream, sender, recipients, timeout_secs,
+            ).await?;
+            (acc, rej)
+        } else {
+            // Sequential: MAIL FROM, then each RCPT TO
+            protocol::send_mail_from(stream, sender, timeout_secs).await?;

-        // RCPT TO for each recipient
-        let mut accepted = Vec::new();
-        let mut rejected = Vec::new();
+            let mut accepted = Vec::new();
+            let mut rejected = Vec::new();

-        for rcpt in recipients {
-            match protocol::send_rcpt_to(stream, rcpt, timeout_secs).await {
-                Ok(resp) => {
-                    if resp.is_success() {
-                        accepted.push(rcpt.clone());
-                    } else {
+            for rcpt in recipients {
+                match protocol::send_rcpt_to(stream, rcpt, timeout_secs).await {
+                    Ok(resp) => {
+                        if resp.is_success() {
+                            accepted.push(rcpt.clone());
+                        } else {
+                            rejected.push(rcpt.clone());
+                        }
+                    }
+                    Err(_) => {
                        rejected.push(rcpt.clone());
                    }
                }
-                Err(_) => {
-                    rejected.push(rcpt.clone());
-                }
            }
-        }
+            (accepted, rejected)
+        };

        // If no recipients were accepted, fail
        if accepted.is_empty() {
@@ -339,6 +350,7 @@ impl SmtpClientManager {
                &config.host,
                config.port,
                config.connection_timeout_secs,
+                config.tls_opportunistic,
            )
            .await?
        } else {
--- a/rust/crates/mailer-smtp/src/client/protocol.rs
+++ b/rust/crates/mailer-smtp/src/client/protocol.rs
@@ -318,6 +318,54 @@ pub async fn send_rcpt_to(
    Ok(resp)
 }

+/// Send MAIL FROM + RCPT TO commands in a single pipelined batch.
+///
+/// Writes all envelope commands at once, then reads responses in order.
+/// Returns `(mail_from_ok, accepted_recipients, rejected_recipients)`.
+pub async fn send_pipelined_envelope(
+    stream: &mut ClientSmtpStream,
+    sender: &str,
+    recipients: &[String],
+    timeout_secs: u64,
+) -> Result<(bool, Vec<String>, Vec<String>), SmtpClientError> {
+    // Build the full pipelined command batch
+    let mut batch = format!("MAIL FROM:<{sender}>\r\n");
+    for rcpt in recipients {
+        batch.push_str(&format!("RCPT TO:<{rcpt}>\r\n"));
+    }
+
+    // Send all commands at once
+    debug!("SMTP C (pipelined): MAIL FROM + {} RCPT TO", recipients.len());
+    stream.write_all(batch.as_bytes()).await?;
+    stream.flush().await?;
+
+    // Read MAIL FROM response
+    let mail_resp = read_response(stream, timeout_secs).await?;
+    if !mail_resp.is_success() {
+        return Err(mail_resp.to_error());
+    }
+
+    // Read RCPT TO responses
+    let mut accepted = Vec::new();
+    let mut rejected = Vec::new();
+    for rcpt in recipients {
+        match read_response(stream, timeout_secs).await {
+            Ok(resp) => {
+                if resp.is_success() {
+                    accepted.push(rcpt.clone());
+                } else {
+                    rejected.push(rcpt.clone());
+                }
+            }
+            Err(_) => {
+                rejected.push(rcpt.clone());
+            }
+        }
+    }
+
+    Ok((true, accepted, rejected))
+}
+
 /// Send DATA command, followed by the message body with dot-stuffing.
 pub async fn send_data(
    stream: &mut ClientSmtpStream,
--- a/rust/crates/mailer-smtp/src/command.rs
+++ b/rust/crates/mailer-smtp/src/command.rs
@@ -50,6 +50,7 @@ pub enum SmtpCommand {
 pub enum AuthMechanism {
    Plain,
    Login,
+    ScramSha256,
 }

 /// Errors that can occur during command parsing.
@@ -218,6 +219,7 @@ fn parse_auth(rest: &str) -> Result<SmtpCommand, ParseError> {
    let mechanism = match mech_str.to_ascii_uppercase().as_str() {
        "PLAIN" => AuthMechanism::Plain,
        "LOGIN" => AuthMechanism::Login,
+        "SCRAM-SHA-256" => AuthMechanism::ScramSha256,
        other => {
            return Err(ParseError::SyntaxError(format!(
                "unsupported AUTH mechanism: {other}"
--- a/rust/crates/mailer-smtp/src/config.rs
+++ b/rust/crates/mailer-smtp/src/config.rs
@@ -2,6 +2,17 @@

 use serde::{Deserialize, Serialize};

+/// Per-domain TLS certificate for SNI-based cert selection.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct TlsDomainCert {
+    /// Domain names this certificate covers (matched against SNI hostname).
+    pub domains: Vec<String>,
+    /// Certificate chain in PEM format.
+    pub cert_pem: String,
+    /// Private key in PEM format.
+    pub key_pem: String,
+}
+
 /// Configuration for an SMTP server instance.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct SmtpServerConfig {
@@ -11,10 +22,13 @@ pub struct SmtpServerConfig {
    pub ports: Vec<u16>,
    /// Port for implicit TLS (e.g. 465). None = no implicit TLS port.
    pub secure_port: Option<u16>,
-    /// TLS certificate chain in PEM format.
+    /// TLS certificate chain in PEM format (default cert).
    pub tls_cert_pem: Option<String>,
-    /// TLS private key in PEM format.
+    /// TLS private key in PEM format (default key).
    pub tls_key_pem: Option<String>,
+    /// Additional per-domain TLS certificates for SNI-based selection.
+    #[serde(default)]
+    pub additional_tls_certs: Vec<TlsDomainCert>,
    /// Maximum message size in bytes.
    pub max_message_size: u64,
    /// Maximum number of concurrent connections.
@@ -43,6 +57,7 @@ impl Default for SmtpServerConfig {
            secure_port: None,
            tls_cert_pem: None,
            tls_key_pem: None,
+            additional_tls_certs: Vec::new(),
            max_message_size: 10 * 1024 * 1024, // 10 MB
            max_connections: 100,
            max_recipients: 100,
--- a/rust/crates/mailer-smtp/src/connection.rs
+++ b/rust/crates/mailer-smtp/src/connection.rs
@@ -9,6 +9,7 @@ use crate::config::SmtpServerConfig;
 use crate::data::{DataAccumulator, DataAction};
 use crate::rate_limiter::RateLimiter;
 use crate::response::{build_capabilities, SmtpResponse};
+use crate::scram::{ScramCredentials, ScramServer};
 use crate::session::{AuthState, SmtpSession};
 use crate::validation;

@@ -52,6 +53,13 @@ pub enum ConnectionEvent {
        password: String,
        remote_addr: String,
    },
+    /// A SCRAM credential request — Rust needs stored credentials from TS.
+    ScramCredentialRequest {
+        correlation_id: String,
+        session_id: String,
+        username: String,
+        remote_addr: String,
+    },
 }

 /// How email data is transported from Rust to TS.
@@ -81,6 +89,16 @@ pub struct AuthResult {
    pub message: Option<String>,
 }

+/// Result of TS returning SCRAM credentials for a user.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ScramCredentialResult {
+    pub found: bool,
+    pub salt: Option<Vec<u8>>,
+    pub iterations: Option<u32>,
+    pub stored_key: Option<Vec<u8>>,
+    pub server_key: Option<Vec<u8>>,
+}
+
 /// Abstraction over plain and TLS streams.
 pub enum SmtpStream {
    Plain(BufReader<TcpStream>),
@@ -133,6 +151,14 @@ impl SmtpStream {
        }
    }

+    /// Check if the internal buffer has unread data (pipelined commands).
+    pub fn has_buffered_data(&self) -> bool {
+        match self {
+            SmtpStream::Plain(reader) => !reader.buffer().is_empty(),
+            SmtpStream::Tls(reader) => !reader.buffer().is_empty(),
+        }
+    }
+
    /// Unwrap to get the raw TcpStream for STARTTLS upgrade.
    /// Only works on Plain streams.
    pub fn into_tcp_stream(self) -> Option<TcpStream> {
@@ -212,7 +238,7 @@ pub async fn handle_connection(
                break;
            }
            Ok(Ok(_)) => {
-                // Process command
+                // Process the first command
                let response = process_line(
                    &line,
                    &mut session,
@@ -227,59 +253,123 @@ pub async fn handle_connection(
                )
                .await;

+                // Check for pipelined commands in the buffer.
+                // Collect pipelinable responses into a batch for single write.
+                let mut response_batch: Vec<u8> = Vec::new();
+                let mut should_break = false;
+                let mut starttls_signal = false;
+
                match response {
                    LineResult::Response(resp) => {
-                        if stream.write_all(&resp.to_bytes()).await.is_err() {
-                            break;
-                        }
-                        if stream.flush().await.is_err() {
-                            break;
-                        }
+                        response_batch.extend_from_slice(&resp.to_bytes());
                    }
                    LineResult::Quit(resp) => {
                        let _ = stream.write_all(&resp.to_bytes()).await;
                        let _ = stream.flush().await;
-                        break;
+                        should_break = true;
                    }
                    LineResult::StartTlsSignal => {
-                        // Send 220 Ready response
-                        let resp = SmtpResponse::new(220, "Ready to start TLS");
-                        if stream.write_all(&resp.to_bytes()).await.is_err() {
-                            break;
-                        }
-                        if stream.flush().await.is_err() {
-                            break;
-                        }
-                        // Extract TCP stream and upgrade
-                        if let Some(tcp_stream) = stream.into_tcp_stream() {
-                            if let Some(acceptor) = &tls_acceptor {
-                                match acceptor.accept(tcp_stream).await {
-                                    Ok(tls_stream) => {
-                                        stream = SmtpStream::Tls(BufReader::new(tls_stream));
-                                        session.secure = true;
-                                        // Client must re-EHLO after STARTTLS
-                                        session.state = crate::state::SmtpState::Connected;
-                                        session.client_hostname = None;
-                                        session.esmtp = false;
-                                        session.auth_state = AuthState::None;
-                                        session.envelope = Default::default();
-                                        debug!(session_id = %session.id, "TLS upgrade successful");
-                                    }
-                                    Err(e) => {
-                                        warn!(session_id = %session.id, error = %e, "TLS handshake failed");
-                                        break;
-                                    }
-                                }
-                            } else {
-                                break;
-                            }
-                        } else {
-                            // Already TLS — shouldn't happen
-                            break;
-                        }
+                        starttls_signal = true;
                    }
                    LineResult::NoResponse => {}
                    LineResult::Disconnect => {
+                        should_break = true;
+                    }
+                }
+
+                if should_break {
+                    break;
+                }
+
+                // Process additional pipelined commands from the buffer
+                if !starttls_signal {
+                    while stream.has_buffered_data() {
+                        let mut next_line = String::new();
+                        match stream.read_line(&mut next_line, 4096).await {
+                            Ok(0) | Err(_) => break,
+                            Ok(_) => {
+                                let next_response = process_line(
+                                    &next_line,
+                                    &mut session,
+                                    &mut stream,
+                                    &config,
+                                    &rate_limiter,
+                                    &event_tx,
+                                    callback_register.as_ref(),
+                                    &tls_acceptor,
+                                    &authenticator,
+                                    &resolver,
+                                )
+                                .await;
+
+                                match next_response {
+                                    LineResult::Response(resp) => {
+                                        response_batch.extend_from_slice(&resp.to_bytes());
+                                    }
+                                    LineResult::Quit(resp) => {
+                                        response_batch.extend_from_slice(&resp.to_bytes());
+                                        should_break = true;
+                                        break;
+                                    }
+                                    LineResult::StartTlsSignal | LineResult::Disconnect => {
+                                        // Non-pipelinable: flush batch and handle
+                                        starttls_signal = matches!(next_response, LineResult::StartTlsSignal);
+                                        should_break = matches!(next_response, LineResult::Disconnect);
+                                        break;
+                                    }
+                                    LineResult::NoResponse => {}
+                                }
+                            }
+                        }
+                    }
+                }
+
+                // Flush the accumulated response batch in one write
+                if !response_batch.is_empty() {
+                    if stream.write_all(&response_batch).await.is_err() {
+                        break;
+                    }
+                    if stream.flush().await.is_err() {
+                        break;
+                    }
+                }
+
+                if should_break {
+                    break;
+                }
+
+                if starttls_signal {
+                    // Send 220 Ready response
+                    let resp = SmtpResponse::new(220, "Ready to start TLS");
+                    if stream.write_all(&resp.to_bytes()).await.is_err() {
+                        break;
+                    }
+                    if stream.flush().await.is_err() {
+                        break;
+                    }
+                    // Extract TCP stream and upgrade
+                    if let Some(tcp_stream) = stream.into_tcp_stream() {
+                        if let Some(acceptor) = &tls_acceptor {
+                            match acceptor.accept(tcp_stream).await {
+                                Ok(tls_stream) => {
+                                    stream = SmtpStream::Tls(BufReader::new(tls_stream));
+                                    session.secure = true;
+                                    session.state = crate::state::SmtpState::Connected;
+                                    session.client_hostname = None;
+                                    session.esmtp = false;
+                                    session.auth_state = AuthState::None;
+                                    session.envelope = Default::default();
+                                    debug!(session_id = %session.id, "TLS upgrade successful");
+                                }
+                                Err(e) => {
+                                    warn!(session_id = %session.id, error = %e, "TLS handshake failed");
+                                    break;
+                                }
+                            }
+                        } else {
+                            break;
+                        }
+                    } else {
                        break;
                    }
                }
@@ -322,6 +412,12 @@ pub trait CallbackRegistry: Send + Sync {
        &self,
        correlation_id: &str,
    ) -> oneshot::Receiver<AuthResult>;
+
+    /// Register a callback for SCRAM credential lookup and return a receiver.
+    fn register_scram_callback(
+        &self,
+        correlation_id: &str,
+    ) -> oneshot::Receiver<ScramCredentialResult>;
 }

 /// Process a single input line from the client.
@@ -406,16 +502,29 @@ async fn process_line(
            mechanism,
            initial_response,
        } => {
-            handle_auth(
-                mechanism,
-                initial_response,
-                session,
-                config,
-                rate_limiter,
-                event_tx,
-                callback_registry,
-            )
-            .await
+            if matches!(mechanism, AuthMechanism::ScramSha256) {
+                handle_auth_scram(
+                    initial_response,
+                    session,
+                    stream,
+                    config,
+                    rate_limiter,
+                    event_tx,
+                    callback_registry,
+                )
+                .await
+            } else {
+                handle_auth(
+                    mechanism,
+                    initial_response,
+                    session,
+                    config,
+                    rate_limiter,
+                    event_tx,
+                    callback_registry,
+                )
+                .await
+            }
        }

        SmtpCommand::Help(_) => {
@@ -832,6 +941,217 @@ async fn handle_auth(
                ))
            }
        }
+        AuthMechanism::ScramSha256 => {
+            // SCRAM is handled separately in process_line; this should not be reached.
+            LineResult::Response(SmtpResponse::not_implemented())
+        }
+    }
+}
+
+/// Handle AUTH SCRAM-SHA-256 — full exchange in a single async function.
+///
+/// SCRAM is a multi-step challenge-response protocol:
+/// 1. Client sends client-first-message (in initial_response or after 334)
+/// 2. Server requests SCRAM credentials from TS
+/// 3. Server sends server-first-message (334 challenge)
+/// 4. Client sends client-final-message (proof)
+/// 5. Server verifies proof and responds with 235 or 535
+async fn handle_auth_scram(
+    initial_response: Option<String>,
+    session: &mut SmtpSession,
+    stream: &mut SmtpStream,
+    config: &SmtpServerConfig,
+    rate_limiter: &RateLimiter,
+    event_tx: &mpsc::Sender<ConnectionEvent>,
+    callback_registry: &dyn CallbackRegistry,
+) -> LineResult {
+    if !config.auth_enabled {
+        return LineResult::Response(SmtpResponse::not_implemented());
+    }
+
+    if session.is_authenticated() {
+        return LineResult::Response(SmtpResponse::bad_sequence("Already authenticated"));
+    }
+
+    if !session.state.can_auth() {
+        return LineResult::Response(SmtpResponse::bad_sequence("Send EHLO first"));
+    }
+
+    // Step 1: Get client-first-message
+    let client_first_b64 = match initial_response {
+        Some(s) if !s.is_empty() => s,
+        _ => {
+            // No initial response — send empty 334 challenge
+            let resp = SmtpResponse::auth_challenge("");
+            if stream.write_all(&resp.to_bytes()).await.is_err() {
+                return LineResult::Disconnect;
+            }
+            if stream.flush().await.is_err() {
+                return LineResult::Disconnect;
+            }
+            // Read client-first-message
+            let mut line = String::new();
+            let socket_timeout = Duration::from_secs(config.socket_timeout_secs);
+            match timeout(socket_timeout, stream.read_line(&mut line, 4096)).await {
+                Err(_) | Ok(Err(_)) | Ok(Ok(0)) => return LineResult::Disconnect,
+                Ok(Ok(_)) => {}
+            }
+            let trimmed = line.trim().to_string();
+            if trimmed == "*" {
+                return LineResult::Response(SmtpResponse::new(501, "Authentication cancelled"));
+            }
+            trimmed
+        }
+    };
+
+    // Decode base64 client-first-message
+    let client_first_bytes = match BASE64.decode(client_first_b64.as_bytes()) {
+        Ok(b) => b,
+        Err(_) => {
+            return LineResult::Response(SmtpResponse::param_error("Invalid base64 encoding"));
+        }
+    };
+    let client_first = match String::from_utf8(client_first_bytes) {
+        Ok(s) => s,
+        Err(_) => {
+            return LineResult::Response(SmtpResponse::param_error("Invalid UTF-8 in SCRAM message"));
+        }
+    };
+
+    // Parse client-first-message
+    let mut scram = match ScramServer::from_client_first(&client_first) {
+        Ok(s) => s,
+        Err(e) => {
+            debug!(error = %e, "SCRAM client-first-message parse error");
+            return LineResult::Response(SmtpResponse::param_error(
+                "Invalid SCRAM client-first-message",
+            ));
+        }
+    };
+
+    // Step 2: Request SCRAM credentials from TS
+    let correlation_id = uuid::Uuid::new_v4().to_string();
+    let rx = callback_registry.register_scram_callback(&correlation_id);
+
+    let event = ConnectionEvent::ScramCredentialRequest {
+        correlation_id: correlation_id.clone(),
+        session_id: session.id.clone(),
+        username: scram.username.clone(),
+        remote_addr: session.remote_addr.clone(),
+    };
+
+    if event_tx.send(event).await.is_err() {
+        return LineResult::Response(SmtpResponse::local_error("Internal processing error"));
+    }
+
+    // Wait for credentials from TS
+    let cred_timeout = Duration::from_secs(5);
+    let cred_result = match timeout(cred_timeout, rx).await {
+        Ok(Ok(result)) => result,
+        Ok(Err(_)) => {
+            warn!(correlation_id = %correlation_id, "SCRAM credential callback dropped");
+            return LineResult::Response(SmtpResponse::local_error("Internal processing error"));
+        }
+        Err(_) => {
+            warn!(correlation_id = %correlation_id, "SCRAM credential request timed out");
+            return LineResult::Response(SmtpResponse::local_error("Internal processing error"));
+        }
+    };
+
+    if !cred_result.found {
+        // User not found — fail auth (don't reveal that user doesn't exist)
+        session.auth_state = AuthState::None;
+        let exceeded = session.record_auth_failure(config.max_auth_failures);
+        if exceeded {
+            return LineResult::Quit(SmtpResponse::service_unavailable(
+                &config.hostname,
+                "Too many authentication failures",
+            ));
+        }
+        return LineResult::Response(SmtpResponse::auth_failed());
+    }
+
+    let creds = ScramCredentials {
+        salt: cred_result.salt.unwrap_or_default(),
+        iterations: cred_result.iterations.unwrap_or(4096),
+        stored_key: cred_result.stored_key.unwrap_or_default(),
+        server_key: cred_result.server_key.unwrap_or_default(),
+    };
+
+    // Step 3: Generate and send server-first-message
+    let server_first = scram.server_first_message(creds);
+    let server_first_b64 = BASE64.encode(server_first.as_bytes());
+
+    let challenge = SmtpResponse::auth_challenge(&server_first_b64);
+    if stream.write_all(&challenge.to_bytes()).await.is_err() {
+        return LineResult::Disconnect;
+    }
+    if stream.flush().await.is_err() {
+        return LineResult::Disconnect;
+    }
+
+    // Step 4: Read client-final-message
+    let mut client_final_line = String::new();
+    let socket_timeout = Duration::from_secs(config.socket_timeout_secs);
+    match timeout(socket_timeout, stream.read_line(&mut client_final_line, 4096)).await {
+        Err(_) | Ok(Err(_)) | Ok(Ok(0)) => return LineResult::Disconnect,
+        Ok(Ok(_)) => {}
+    }
+
+    let client_final_b64 = client_final_line.trim();
+
+    // Cancel if *
+    if client_final_b64 == "*" {
+        session.auth_state = AuthState::None;
+        return LineResult::Response(SmtpResponse::new(501, "Authentication cancelled"));
+    }
+
+    // Decode base64 client-final-message
+    let client_final_bytes = match BASE64.decode(client_final_b64.as_bytes()) {
+        Ok(b) => b,
+        Err(_) => {
+            session.auth_state = AuthState::None;
+            return LineResult::Response(SmtpResponse::param_error("Invalid base64 encoding"));
+        }
+    };
+    let client_final = match String::from_utf8(client_final_bytes) {
+        Ok(s) => s,
+        Err(_) => {
+            session.auth_state = AuthState::None;
+            return LineResult::Response(SmtpResponse::param_error("Invalid UTF-8 in SCRAM message"));
+        }
+    };
+
+    // Step 5: Verify proof
+    match scram.process_client_final(&client_final) {
+        Ok(server_final) => {
+            let server_final_b64 = BASE64.encode(server_final.as_bytes());
+            session.auth_state = AuthState::Authenticated {
+                username: scram.username.clone(),
+            };
+            LineResult::Response(SmtpResponse::new(
+                235,
+                format!("2.7.0 Authentication successful {}", server_final_b64),
+            ))
+        }
+        Err(e) => {
+            debug!(error = %e, "SCRAM proof verification failed");
+            session.auth_state = AuthState::None;
+            let exceeded = session.record_auth_failure(config.max_auth_failures);
+            if exceeded {
+                if !rate_limiter.check_auth_failure(&session.remote_addr) {
+                    return LineResult::Quit(SmtpResponse::service_unavailable(
+                        &config.hostname,
+                        "Too many authentication failures",
+                    ));
+                }
+                return LineResult::Quit(SmtpResponse::service_unavailable(
+                    &config.hostname,
+                    "Too many authentication failures",
+                ));
+            }
+            LineResult::Response(SmtpResponse::auth_failed())
+        }
    }
 }

--- a/rust/crates/mailer-smtp/src/lib.rs
+++ b/rust/crates/mailer-smtp/src/lib.rs
@@ -19,6 +19,7 @@ pub mod connection;
 pub mod data;
 pub mod rate_limiter;
 pub mod response;
+pub mod scram;
 pub mod server;
 pub mod session;
 pub mod state;
--- a/rust/crates/mailer-smtp/src/response.rs
+++ b/rust/crates/mailer-smtp/src/response.rs
@@ -196,7 +196,7 @@ pub fn build_capabilities(
        caps.push("STARTTLS".to_string());
    }
    if auth_available {
-        caps.push("AUTH PLAIN LOGIN".to_string());
+        caps.push("AUTH PLAIN LOGIN SCRAM-SHA-256".to_string());
    }
    caps
 }
@@ -253,7 +253,7 @@ mod tests {
        let caps = build_capabilities(10485760, true, false, true);
        assert!(caps.contains(&"SIZE 10485760".to_string()));
        assert!(caps.contains(&"STARTTLS".to_string()));
-        assert!(caps.contains(&"AUTH PLAIN LOGIN".to_string()));
+        assert!(caps.contains(&"AUTH PLAIN LOGIN SCRAM-SHA-256".to_string()));
        assert!(caps.contains(&"PIPELINING".to_string()));
    }

@@ -262,7 +262,7 @@ mod tests {
        // When already secure, STARTTLS should NOT be advertised
        let caps = build_capabilities(10485760, true, true, false);
        assert!(!caps.contains(&"STARTTLS".to_string()));
-        assert!(!caps.contains(&"AUTH PLAIN LOGIN".to_string()));
+        assert!(!caps.contains(&"AUTH PLAIN LOGIN SCRAM-SHA-256".to_string()));
    }

    #[test]
--- a/rust/crates/mailer-smtp/src/scram.rs
+++ b/rust/crates/mailer-smtp/src/scram.rs
@@ -0,0 +1,342 @@
+//! SCRAM-SHA-256 server-side implementation (RFC 5802 + RFC 7677).
+//!
+//! Implements the server side of the SCRAM-SHA-256 SASL mechanism,
+//! a challenge-response protocol that avoids transmitting cleartext passwords.
+
+use base64::engine::general_purpose::STANDARD as BASE64;
+use base64::Engine;
+use hmac::{Hmac, Mac};
+use sha2::{Digest, Sha256};
+
+type HmacSha256 = Hmac<Sha256>;
+
+/// Pre-computed SCRAM credentials for a user (derived from password).
+#[derive(Debug, Clone)]
+pub struct ScramCredentials {
+    pub salt: Vec<u8>,
+    pub iterations: u32,
+    pub stored_key: Vec<u8>,
+    pub server_key: Vec<u8>,
+}
+
+/// Server-side SCRAM state machine.
+pub struct ScramServer {
+    /// Username extracted from client-first-message.
+    pub username: String,
+    /// Full combined nonce (client + server).
+    combined_nonce: String,
+    /// Server nonce portion (used in tests for verification).
+    #[allow(dead_code)]
+    server_nonce: String,
+    /// Stored credentials (set after TS responds).
+    credentials: Option<ScramCredentials>,
+    /// The server-first-message (for auth message construction).
+    server_first: String,
+    /// The client-first-message-bare (for auth message construction).
+    client_first_bare: String,
+}
+
+impl ScramServer {
+    /// Process the client-first-message.
+    ///
+    /// Parses the client nonce and username, generates a server nonce,
+    /// and returns a partial state that needs credentials to produce the
+    /// server-first-message.
+    pub fn from_client_first(client_first: &str) -> Result<Self, String> {
+        // client-first-message = gs2-header client-first-message-bare
+        // gs2-header = "n,," (no channel binding)
+        // client-first-message-bare = "n=username,r=nonce"
+        let bare = if let Some(rest) = client_first.strip_prefix("n,,") {
+            rest
+        } else if let Some(rest) = client_first.strip_prefix("y,,") {
+            rest
+        } else {
+            return Err("Invalid SCRAM gs2-header".into());
+        };
+
+        let mut username = String::new();
+        let mut client_nonce = String::new();
+
+        for part in bare.split(',') {
+            if let Some(val) = part.strip_prefix("n=") {
+                username = val.to_string();
+            } else if let Some(val) = part.strip_prefix("r=") {
+                client_nonce = val.to_string();
+            }
+        }
+
+        if username.is_empty() || client_nonce.is_empty() {
+            return Err("Missing username or nonce in client-first-message".into());
+        }
+
+        // Generate server nonce
+        let server_nonce: String = (0..24)
+            .map(|_| {
+                let idx = (rand_byte() as usize) % 62;
+                b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"[idx] as char
+            })
+            .collect();
+
+        let combined_nonce = format!("{}{}", client_nonce, server_nonce);
+
+        Ok(ScramServer {
+            username,
+            combined_nonce,
+            server_nonce,
+            credentials: None,
+            server_first: String::new(),
+            client_first_bare: bare.to_string(),
+        })
+    }
+
+    /// Set the credentials and produce the server-first-message.
+    pub fn server_first_message(&mut self, creds: ScramCredentials) -> String {
+        let salt_b64 = BASE64.encode(&creds.salt);
+        let server_first = format!(
+            "r={},s={},i={}",
+            self.combined_nonce, salt_b64, creds.iterations
+        );
+
+        self.server_first = server_first.clone();
+        self.credentials = Some(creds);
+        server_first
+    }
+
+    /// Process the client-final-message and verify the proof.
+    ///
+    /// Returns the server-final-message (containing ServerSignature) on success,
+    /// or an error string on failure.
+    pub fn process_client_final(&mut self, client_final: &str) -> Result<String, String> {
+        let creds = self.credentials.as_ref().ok_or("No credentials set")?;
+
+        // Parse client-final-message
+        // Format: c=biws,r=<combined_nonce>,p=<client_proof>
+        let mut channel_binding = String::new();
+        let mut nonce = String::new();
+        let mut proof_b64 = String::new();
+
+        for part in client_final.split(',') {
+            if let Some(val) = part.strip_prefix("c=") {
+                channel_binding = val.to_string();
+            } else if let Some(val) = part.strip_prefix("r=") {
+                nonce = val.to_string();
+            } else if let Some(val) = part.strip_prefix("p=") {
+                proof_b64 = val.to_string();
+            }
+        }
+
+        // Verify nonce matches
+        if nonce != self.combined_nonce {
+            return Err("Nonce mismatch".into());
+        }
+
+        // Build the client-final-message-without-proof
+        let client_final_without_proof = format!("c={},r={}", channel_binding, nonce);
+
+        // Complete the auth message
+        let auth_message = format!(
+            "{},{},{}",
+            self.client_first_bare, self.server_first, client_final_without_proof
+        );
+
+        // Verify client proof
+        let client_proof = BASE64.decode(proof_b64.as_bytes())
+            .map_err(|_| "Invalid base64 in client proof")?;
+
+        // ClientSignature = HMAC(StoredKey, AuthMessage)
+        let client_signature = hmac_sha256(&creds.stored_key, auth_message.as_bytes());
+
+        // ClientKey = ClientProof XOR ClientSignature
+        if client_proof.len() != client_signature.len() {
+            return Err("Client proof length mismatch".into());
+        }
+        let client_key: Vec<u8> = client_proof
+            .iter()
+            .zip(client_signature.iter())
+            .map(|(a, b)| a ^ b)
+            .collect();
+
+        // StoredKey = H(ClientKey)
+        let computed_stored_key = sha256(&client_key);
+
+        // Verify: computed StoredKey must match the stored StoredKey
+        if computed_stored_key != creds.stored_key {
+            return Err("Authentication failed".into());
+        }
+
+        // Generate ServerSignature for mutual authentication
+        let server_signature = hmac_sha256(&creds.server_key, auth_message.as_bytes());
+        let server_sig_b64 = BASE64.encode(&server_signature);
+
+        Ok(format!("v={}", server_sig_b64))
+    }
+}
+
+/// Compute SCRAM credentials from a plaintext password (for TS to pre-compute).
+pub fn compute_scram_credentials(password: &str, salt: &[u8], iterations: u32) -> ScramCredentials {
+    // SaltedPassword = PBKDF2-HMAC-SHA256(password, salt, iterations)
+    let mut salted_password = [0u8; 32];
+    pbkdf2::pbkdf2_hmac::<Sha256>(
+        password.as_bytes(),
+        salt,
+        iterations,
+        &mut salted_password,
+    );
+
+    // ClientKey = HMAC(SaltedPassword, "Client Key")
+    let client_key = hmac_sha256(&salted_password, b"Client Key");
+
+    // StoredKey = H(ClientKey)
+    let stored_key = sha256(&client_key);
+
+    // ServerKey = HMAC(SaltedPassword, "Server Key")
+    let server_key = hmac_sha256(&salted_password, b"Server Key");
+
+    ScramCredentials {
+        salt: salt.to_vec(),
+        iterations,
+        stored_key,
+        server_key,
+    }
+}
+
+fn hmac_sha256(key: &[u8], data: &[u8]) -> Vec<u8> {
+    let mut mac = HmacSha256::new_from_slice(key).expect("HMAC accepts any key length");
+    mac.update(data);
+    mac.finalize().into_bytes().to_vec()
+}
+
+fn sha256(data: &[u8]) -> Vec<u8> {
+    let mut hasher = Sha256::new();
+    hasher.update(data);
+    hasher.finalize().to_vec()
+}
+
+/// Simple random byte using system randomness.
+fn rand_byte() -> u8 {
+    use std::collections::hash_map::RandomState;
+    use std::hash::{BuildHasher, Hasher};
+    let state = RandomState::new();
+    let mut hasher = state.build_hasher();
+    hasher.write_u64(std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_nanos() as u64);
+    hasher.finish() as u8
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_scram_full_exchange() {
+        let password = "pencil";
+        let salt = b"test-salt-1234";
+        let iterations = 4096;
+
+        // Pre-compute server-side credentials from password
+        let creds = compute_scram_credentials(password, salt, iterations);
+
+        // 1. Client sends client-first-message
+        let client_first = "n,,n=user,r=rOprNGfwEbeRWgbNEkqO";
+        let mut server = ScramServer::from_client_first(client_first).unwrap();
+        assert_eq!(server.username, "user");
+
+        // 2. Server responds with server-first-message
+        let server_first = server.server_first_message(creds.clone());
+        assert!(server_first.starts_with(&format!("r=rOprNGfwEbeRWgbNEkqO{}", server.server_nonce)));
+        assert!(server_first.contains("s="));
+        assert!(server_first.contains("i=4096"));
+
+        // 3. Client computes proof
+        // SaltedPassword
+        let mut salted_password = [0u8; 32];
+        pbkdf2::pbkdf2_hmac::<Sha256>(
+            password.as_bytes(),
+            salt,
+            iterations,
+            &mut salted_password,
+        );
+
+        let client_key = hmac_sha256(&salted_password, b"Client Key");
+        let stored_key = sha256(&client_key);
+
+        let client_first_bare = "n=user,r=rOprNGfwEbeRWgbNEkqO";
+        let client_final_without_proof = format!("c=biws,r={}", server.combined_nonce);
+        let auth_message = format!("{},{},{}", client_first_bare, server_first, client_final_without_proof);
+
+        let client_signature = hmac_sha256(&stored_key, auth_message.as_bytes());
+        let client_proof: Vec<u8> = client_key
+            .iter()
+            .zip(client_signature.iter())
+            .map(|(a, b)| a ^ b)
+            .collect();
+        let proof_b64 = BASE64.encode(&client_proof);
+
+        let client_final = format!("c=biws,r={},p={}", server.combined_nonce, proof_b64);
+
+        // 4. Server verifies proof
+        let result = server.process_client_final(&client_final);
+        assert!(result.is_ok(), "SCRAM verification failed: {:?}", result.err());
+        let server_final = result.unwrap();
+        assert!(server_final.starts_with("v="));
+    }
+
+    #[test]
+    fn test_scram_wrong_password() {
+        let password = "pencil";
+        let wrong_password = "wrong";
+        let salt = b"test-salt";
+        let iterations = 4096;
+
+        let creds = compute_scram_credentials(password, salt, iterations);
+
+        let client_first = "n,,n=user,r=clientnonce123";
+        let mut server = ScramServer::from_client_first(client_first).unwrap();
+        let server_first = server.server_first_message(creds);
+
+        // Client computes proof with wrong password
+        let mut salted_password = [0u8; 32];
+        pbkdf2::pbkdf2_hmac::<Sha256>(
+            wrong_password.as_bytes(),
+            salt,
+            iterations,
+            &mut salted_password,
+        );
+
+        let client_key = hmac_sha256(&salted_password, b"Client Key");
+        let stored_key = sha256(&client_key);
+
+        let client_first_bare = "n=user,r=clientnonce123";
+        let client_final_without_proof = format!("c=biws,r={}", server.combined_nonce);
+        let auth_message = format!("{},{},{}", client_first_bare, server_first, client_final_without_proof);
+
+        let client_signature = hmac_sha256(&stored_key, auth_message.as_bytes());
+        let client_proof: Vec<u8> = client_key
+            .iter()
+            .zip(client_signature.iter())
+            .map(|(a, b)| a ^ b)
+            .collect();
+        let proof_b64 = BASE64.encode(&client_proof);
+
+        let client_final = format!("c=biws,r={},p={}", server.combined_nonce, proof_b64);
+        let result = server.process_client_final(&client_final);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_compute_scram_credentials() {
+        let creds = compute_scram_credentials("password", b"salt", 4096);
+        assert_eq!(creds.salt, b"salt");
+        assert_eq!(creds.iterations, 4096);
+        assert_eq!(creds.stored_key.len(), 32);
+        assert_eq!(creds.server_key.len(), 32);
+    }
+
+    #[test]
+    fn test_invalid_client_first() {
+        assert!(ScramServer::from_client_first("invalid").is_err());
+        assert!(ScramServer::from_client_first("n,,").is_err());
+    }
+}
--- a/rust/crates/mailer-smtp/src/server.rs
+++ b/rust/crates/mailer-smtp/src/server.rs
@@ -12,6 +12,7 @@ use crate::rate_limiter::{RateLimitConfig, RateLimiter};
 use hickory_resolver::TokioResolver;
 use mailer_security::MessageAuthenticator;
 use rustls_pki_types::{CertificateDer, PrivateKeyDer};
+use std::collections::HashMap;
 use std::io::BufReader;
 use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
 use std::sync::Arc;
@@ -263,6 +264,69 @@ async fn accept_loop(
    }
 }

+/// SNI-based certificate resolver that selects the appropriate TLS certificate
+/// based on the client's requested hostname.
+struct SniCertResolver {
+    /// Domain -> certified key mapping.
+    certs: HashMap<String, Arc<rustls::sign::CertifiedKey>>,
+    /// Default certificate for non-matching SNI or missing SNI.
+    default: Arc<rustls::sign::CertifiedKey>,
+}
+
+impl std::fmt::Debug for SniCertResolver {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("SniCertResolver")
+            .field("domains", &self.certs.keys().collect::<Vec<_>>())
+            .finish()
+    }
+}
+
+impl rustls::server::ResolvesServerCert for SniCertResolver {
+    fn resolve(
+        &self,
+        client_hello: rustls::server::ClientHello<'_>,
+    ) -> Option<Arc<rustls::sign::CertifiedKey>> {
+        if let Some(sni) = client_hello.server_name() {
+            let sni_lower = sni.to_lowercase();
+            if let Some(key) = self.certs.get(&sni_lower) {
+                return Some(key.clone());
+            }
+        }
+        Some(self.default.clone())
+    }
+}
+
+/// Parse a PEM cert+key pair into a `CertifiedKey`.
+fn parse_certified_key(
+    cert_pem: &str,
+    key_pem: &str,
+) -> Result<rustls::sign::CertifiedKey, Box<dyn std::error::Error + Send + Sync>> {
+    let certs: Vec<CertificateDer<'static>> = {
+        let mut reader = BufReader::new(cert_pem.as_bytes());
+        rustls_pemfile::certs(&mut reader).collect::<Result<Vec<_>, _>>()?
+    };
+    if certs.is_empty() {
+        return Err("No certificates found in PEM".into());
+    }
+
+    let key: PrivateKeyDer<'static> = {
+        let mut reader = BufReader::new(key_pem.as_bytes());
+        let mut keys = Vec::new();
+        for item in rustls_pemfile::read_all(&mut reader) {
+            match item? {
+                rustls_pemfile::Item::Pkcs8Key(key) => keys.push(PrivateKeyDer::Pkcs8(key)),
+                rustls_pemfile::Item::Pkcs1Key(key) => keys.push(PrivateKeyDer::Pkcs1(key)),
+                rustls_pemfile::Item::Sec1Key(key) => keys.push(PrivateKeyDer::Sec1(key)),
+                _ => {}
+            }
+        }
+        keys.into_iter().next().ok_or("No private key found in PEM")?
+    };
+
+    let signing_key = rustls::crypto::ring::sign::any_supported_type(&key)?;
+    Ok(rustls::sign::CertifiedKey::new(certs, signing_key))
+}
+
 /// Build a TLS acceptor from PEM cert/key strings.
 fn build_tls_acceptor(
    config: &SmtpServerConfig,
@@ -311,9 +375,42 @@ fn build_tls_acceptor(
            .ok_or("No private key found in PEM")?
    };

-    let tls_config = rustls::ServerConfig::builder()
-        .with_no_client_auth()
-        .with_single_cert(certs, key)?;
+    // If additional TLS certs are configured, use SNI-based resolution
+    let tls_config = if config.additional_tls_certs.is_empty() {
+        rustls::ServerConfig::builder()
+            .with_no_client_auth()
+            .with_single_cert(certs, key)?
+    } else {
+        // Build default certified key
+        let signing_key = rustls::crypto::ring::sign::any_supported_type(&key)?;
+        let default_ck = Arc::new(rustls::sign::CertifiedKey::new(certs, signing_key));
+
+        // Build per-domain certs
+        let mut domain_certs = HashMap::new();
+        for domain_cert in &config.additional_tls_certs {
+            match parse_certified_key(&domain_cert.cert_pem, &domain_cert.key_pem) {
+                Ok(ck) => {
+                    let ck = Arc::new(ck);
+                    for domain in &domain_cert.domains {
+                        domain_certs.insert(domain.to_lowercase(), ck.clone());
+                    }
+                    info!("SNI cert loaded for domains: {:?}", domain_cert.domains);
+                }
+                Err(e) => {
+                    warn!("Failed to load SNI cert for domains {:?}: {}", domain_cert.domains, e);
+                }
+            }
+        }
+
+        let resolver = SniCertResolver {
+            certs: domain_certs,
+            default: default_ck,
+        };
+
+        rustls::ServerConfig::builder()
+            .with_no_client_auth()
+            .with_cert_resolver(Arc::new(resolver))
+    };

    Ok(tokio_rustls::TlsAcceptor::from(Arc::new(tls_config)))
 }
--- a/test/test.mta.delivery.node.ts
+++ b/test/test.mta.delivery.node.ts
@@ -0,0 +1,295 @@
+import { tap, expect } from '@git.zone/tstest/tapbundle';
+import { UnifiedEmailServer } from '../ts/mail/routing/classes.unified.email.server.js';
+import { RustSecurityBridge } from '../ts/security/classes.rustsecuritybridge.js';
+import { Email } from '../ts/mail/core/classes.email.js';
+import * as net from 'net';
+import * as dns from 'dns';
+
+const storageMap = new Map<string, string>();
+const mockDcRouter = {
+  storageManager: {
+    get: async (key: string) => storageMap.get(key) || null,
+    set: async (key: string, value: string) => { storageMap.set(key, value); },
+  },
+};
+
+let server: UnifiedEmailServer;
+let bridgeAvailable = false;
+let mockSmtpServer: net.Server;
+
+/**
+ * Create a minimal mock SMTP server that accepts any email.
+ */
+function createMockSmtpServer(port: number): Promise<net.Server> {
+  return new Promise((resolve, reject) => {
+    const srv = net.createServer((socket) => {
+      socket.write('220 mock-smtp.local ESMTP MockServer\r\n');
+
+      let inData = false;
+      let dataBuffer = '';
+
+      socket.on('data', (chunk) => {
+        const input = chunk.toString();
+
+        if (inData) {
+          dataBuffer += input;
+          if (dataBuffer.includes('\r\n.\r\n')) {
+            inData = false;
+            dataBuffer = '';
+            socket.write('250 2.0.0 Ok: queued\r\n');
+          }
+          return;
+        }
+
+        const lines = input.split('\r\n').filter((l: string) => l.length > 0);
+        for (const line of lines) {
+          const cmd = line.toUpperCase();
+          if (cmd.startsWith('EHLO') || cmd.startsWith('HELO')) {
+            socket.write(`250-mock-smtp.local\r\n250-SIZE 10485760\r\n250 OK\r\n`);
+          } else if (cmd.startsWith('MAIL FROM')) {
+            socket.write('250 2.1.0 Ok\r\n');
+          } else if (cmd.startsWith('RCPT TO')) {
+            socket.write('250 2.1.5 Ok\r\n');
+          } else if (cmd === 'DATA') {
+            inData = true;
+            dataBuffer = '';
+            socket.write('354 End data with <CR><LF>.<CR><LF>\r\n');
+          } else if (cmd === 'QUIT') {
+            socket.write('221 2.0.0 Bye\r\n');
+            socket.end();
+          } else if (cmd === 'RSET') {
+            socket.write('250 2.0.0 Ok\r\n');
+          }
+        }
+      });
+    });
+
+    srv.listen(port, '127.0.0.1', () => {
+      resolve(srv);
+    });
+
+    srv.on('error', reject);
+  });
+}
+
+// Store original resolveMx so we can restore it
+const originalResolveMx = dns.promises.Resolver.prototype.resolveMx;
+
+tap.test('setup - start server and mock SMTP', async () => {
+  RustSecurityBridge.resetInstance();
+
+  server = new UnifiedEmailServer(mockDcRouter, {
+    ports: [10425],
+    hostname: 'test.mta.local',
+    domains: [
+      { domain: 'mta-test.com', dnsMode: 'forward' },
+    ],
+    routes: [
+      {
+        name: 'mta-route',
+        priority: 10,
+        match: { recipients: '*@mta-test.com' },
+        action: { type: 'deliver' },
+      },
+      {
+        name: 'process-route',
+        priority: 20,
+        match: { recipients: '*@process-test.com' },
+        action: {
+          type: 'process',
+          options: {
+            contentScanning: true,
+            scanners: [{ type: 'spam' }],
+          },
+        },
+      },
+    ],
+  });
+
+  try {
+    await server.start();
+    bridgeAvailable = true;
+  } catch (err) {
+    console.log(`SKIP: Server failed to start — ${(err as Error).message}`);
+    console.log('Build the Rust binary with: cd rust && cargo build --release');
+    return;
+  }
+
+  mockSmtpServer = await createMockSmtpServer(10426);
+});
+
+tap.test('MX resolution for a public domain', async () => {
+  if (!bridgeAvailable) { console.log('SKIP'); return; }
+
+  // Use the delivery system's resolveMxForDomain via a quick DNS lookup
+  const resolver = new dns.promises.Resolver();
+  try {
+    const records = await resolver.resolveMx('gmail.com');
+    expect(records).toBeTruthy();
+    expect(records.length).toBeGreaterThan(0);
+    // Each record should have exchange and priority
+    for (const rec of records) {
+      expect(typeof rec.exchange).toEqual('string');
+      expect(typeof rec.priority).toEqual('number');
+    }
+    console.log(`Resolved ${records.length} MX records for gmail.com`);
+  } catch (err) {
+    console.log(`SKIP: DNS resolution failed (network may be unavailable): ${(err as Error).message}`);
+  }
+});
+
+tap.test('group recipients by domain', async () => {
+  if (!bridgeAvailable) { console.log('SKIP'); return; }
+
+  // Test the grouping logic directly
+  const recipients = [
+    'alice@example.com',
+    'bob@example.com',
+    'carol@other.org',
+    'dave@example.com',
+    'eve@other.org',
+  ];
+
+  const groups = new Map<string, string[]>();
+  for (const rcpt of recipients) {
+    const domain = rcpt.split('@')[1]?.toLowerCase();
+    if (!domain) continue;
+    const list = groups.get(domain) || [];
+    list.push(rcpt);
+    groups.set(domain, list);
+  }
+
+  expect(groups.size).toEqual(2);
+  expect(groups.get('example.com')!.length).toEqual(3);
+  expect(groups.get('other.org')!.length).toEqual(2);
+});
+
+tap.test('MTA delivery to mock SMTP server via mocked MX', async () => {
+  if (!bridgeAvailable) { console.log('SKIP'); return; }
+
+  // Mock dns.promises.Resolver.resolveMx to return 127.0.0.1
+  dns.promises.Resolver.prototype.resolveMx = async function (_hostname: string) {
+    return [{ exchange: '127.0.0.1', priority: 10 }];
+  };
+
+  const email = new Email({
+    from: 'sender@mta-test.com',
+    to: 'recipient@target-domain.com',
+    subject: 'MTA Delivery Test',
+    text: 'Testing MTA delivery with MX resolution.',
+  });
+
+  // Use sendOutboundEmail at the resolved MX host (which is mocked to 127.0.0.1)
+  // But the real test is the delivery system's handleMtaDelivery, which we test
+  // by sending through the server's outbound path with the mock MX.
+
+  // Direct test: resolve MX then send
+  const resolver = new dns.promises.Resolver();
+  const mxRecords = await resolver.resolveMx('target-domain.com');
+  expect(mxRecords[0].exchange).toEqual('127.0.0.1');
+
+  // Send via the resolved MX host to the mock SMTP server on port 10425
+  // Note: MTA delivery uses port 25 by default, but our mock is on 10425.
+  // We test the sendOutboundEmail path directly with the mock MX host.
+  const result = await server.sendOutboundEmail('127.0.0.1', 10426, email);
+  expect(result).toBeTruthy();
+  expect(result.accepted.length).toBeGreaterThan(0);
+  expect(result.response).toInclude('2.0.0');
+
+  // Restore original resolveMx
+  dns.promises.Resolver.prototype.resolveMx = originalResolveMx;
+});
+
+tap.test('MTA delivery - connection refused to unreachable MX', async () => {
+  if (!bridgeAvailable) { console.log('SKIP'); return; }
+
+  const email = new Email({
+    from: 'sender@mta-test.com',
+    to: 'recipient@unreachable-domain.com',
+    subject: 'Connection Refused MX Test',
+    text: 'This should fail — no server at the target.',
+  });
+
+  // Send to a port that nothing is listening on
+  try {
+    await server.sendOutboundEmail('127.0.0.1', 59789, email);
+    throw new Error('Expected sendOutboundEmail to fail');
+  } catch (err: any) {
+    expect(err).toBeTruthy();
+    expect(err.message.length).toBeGreaterThan(0);
+    console.log(`Got expected error: ${err.message}`);
+  }
+});
+
+tap.test('MTA delivery with multiple recipients across domains', async () => {
+  if (!bridgeAvailable) { console.log('SKIP'); return; }
+
+  // Mock MX to return 127.0.0.1 for all domains
+  dns.promises.Resolver.prototype.resolveMx = async function (_hostname: string) {
+    return [{ exchange: '127.0.0.1', priority: 10 }];
+  };
+
+  const email = new Email({
+    from: 'sender@mta-test.com',
+    to: ['alice@domain-a.com', 'bob@domain-b.com'],
+    subject: 'Multi-Domain MTA Test',
+    text: 'Testing delivery to multiple domains.',
+  });
+
+  // Send to each recipient's domain individually (simulating MTA behavior)
+  for (const recipient of email.to) {
+    const singleEmail = new Email({
+      from: email.from,
+      to: recipient,
+      subject: email.subject,
+      text: email.text,
+    });
+    const result = await server.sendOutboundEmail('127.0.0.1', 10426, singleEmail);
+    expect(result.accepted.length).toEqual(1);
+  }
+
+  // Restore original resolveMx
+  dns.promises.Resolver.prototype.resolveMx = originalResolveMx;
+});
+
+tap.test('E2E: send real email to hello@task.vc via MX resolution', async () => {
+  if (!bridgeAvailable) { console.log('SKIP'); return; }
+
+  // Resolve real MX records for task.vc
+  const resolver = new dns.promises.Resolver();
+  const mxRecords = await resolver.resolveMx('task.vc');
+  expect(mxRecords.length).toBeGreaterThan(0);
+  const mxHost = mxRecords.sort((a, b) => a.priority - b.priority)[0].exchange;
+  console.log(`Resolved MX for task.vc: ${mxHost} (priority ${mxRecords[0].priority})`);
+
+  const email = new Email({
+    from: 'test@mta-test.com',
+    to: 'hello@task.vc',
+    subject: `MTA E2E Test — ${new Date().toISOString()}`,
+    text: 'This is an automated E2E test from @serve.zone/mailer verifying real MX resolution and outbound SMTP delivery.',
+  });
+
+  const result = await server.sendOutboundEmail(mxHost, 25, email);
+  expect(result).toBeTruthy();
+  expect(result.accepted).toBeTruthy();
+  expect(result.accepted.length).toEqual(1);
+  expect(result.accepted[0]).toEqual('hello@task.vc');
+  expect(result.response).toInclude('2.0.0');
+  console.log(`Email delivered to hello@task.vc via ${mxHost}: ${result.response}`);
+});
+
+tap.test('cleanup - stop server and mock SMTP', async () => {
+  // Restore MX resolver in case it wasn't restored
+  dns.promises.Resolver.prototype.resolveMx = originalResolveMx;
+
+  // Force-close mock server (destroy all open sockets)
+  if (mockSmtpServer) {
+    mockSmtpServer.close();
+  }
+  if (bridgeAvailable) {
+    await server.stop();
+  }
+  await tap.stopForcefully();
+});
+
+export default tap.start();
--- a/ts/00_commitinfo_data.ts
+++ b/ts/00_commitinfo_data.ts
@@ -3,6 +3,6 @@
 */
 export const commitinfo = {
  name: '@push.rocks/smartmta',
-  version: '5.0.0',
+  version: '5.1.2',
  description: 'A high-performance, enterprise-grade Mail Transfer Agent (MTA) built from scratch in TypeScript with Rust acceleration.'
 }
--- a/ts/mail/delivery/classes.delivery.system.ts
+++ b/ts/mail/delivery/classes.delivery.system.ts
@@ -7,10 +7,12 @@ import {
  SecurityEventType
 } from '../../security/index.js';
 import { UnifiedDeliveryQueue, type IQueueItem } from './classes.delivery.queue.js';
-import type { Email } from '../core/classes.email.js';
+import { Email } from '../core/classes.email.js';
 import type { UnifiedEmailServer } from '../routing/classes.unified.email.server.js';
 import { RustSecurityBridge } from '../../security/classes.rustsecuritybridge.js';

+const dns = plugins.dns;
+
 /**
 * Delivery status enumeration
 */
@@ -480,6 +482,36 @@ export class MultiModeDeliverySystem extends EventEmitter {
    }
  }
  
+  /**
+   * Resolve MX records for a domain, sorted by priority (lowest first).
+   * Falls back to the domain itself as an A record per RFC 5321.
+   */
+  private async resolveMxForDomain(domain: string): Promise<Array<{ exchange: string; priority: number }>> {
+    const resolver = new dns.promises.Resolver();
+    try {
+      const mxRecords = await resolver.resolveMx(domain);
+      return mxRecords.sort((a, b) => a.priority - b.priority);
+    } catch (err) {
+      logger.log('warn', `No MX records for ${domain}, falling back to A record`);
+      return [{ exchange: domain, priority: 0 }];
+    }
+  }
+
+  /**
+   * Group recipient addresses by their domain part.
+   */
+  private groupRecipientsByDomain(recipients: string[]): Map<string, string[]> {
+    const groups = new Map<string, string[]>();
+    for (const rcpt of recipients) {
+      const domain = rcpt.split('@')[1]?.toLowerCase();
+      if (!domain) continue;
+      const list = groups.get(domain) || [];
+      list.push(rcpt);
+      groups.set(domain, list);
+    }
+    return groups;
+  }
+
  /**
   * Default handler for MTA mode delivery
   * @param item Queue item
@@ -488,31 +520,81 @@ export class MultiModeDeliverySystem extends EventEmitter {
    logger.log('info', `MTA delivery for item ${item.id}`);

    const email = item.processingResult as Email;
-    const route = item.route;

-    try {
-      // Apply DKIM signing if configured in the route
-      if (item.route?.action.options?.mtaOptions?.dkimSign) {
-        await this.applyDkimSigning(email, item.route.action.options.mtaOptions);
+    if (!this.emailServer) {
+      throw new Error('No email server available for MTA delivery');
+    }
+
+    // Build DKIM options from route config
+    const dkimDomain = item.route?.action.options?.mtaOptions?.dkimSign
+      ? (item.route.action.options.mtaOptions.dkimOptions?.domainName || email.from.split('@')[1])
+      : undefined;
+    const dkimSelector = item.route?.action.options?.mtaOptions?.dkimOptions?.keySelector || 'default';
+
+    const allRecipients = email.getAllRecipients();
+    if (allRecipients.length === 0) {
+      throw new Error('No recipients specified for MTA delivery');
+    }
+
+    const domainGroups = this.groupRecipientsByDomain(allRecipients);
+    const results: Array<{ domain: string; success: boolean; error?: string; accepted?: string[]; rejected?: string[] }> = [];
+
+    for (const [domain, recipients] of domainGroups) {
+      const mxHosts = await this.resolveMxForDomain(domain);
+      let delivered = false;
+      let lastError: string | undefined;
+
+      for (const mx of mxHosts) {
+        try {
+          logger.log('info', `MTA: trying MX ${mx.exchange}:25 for domain ${domain} (priority ${mx.priority})`);
+
+          // Create a temporary Email scoped to this domain's recipients
+          const domainEmail = new Email({
+            from: email.from,
+            to: recipients.filter(r => email.to.includes(r)),
+            cc: recipients.filter(r => (email.cc || []).includes(r)),
+            bcc: recipients.filter(r => (email.bcc || []).includes(r)),
+            subject: email.subject,
+            text: email.text,
+            html: email.html,
+          });
+
+          const result = await this.emailServer.sendOutboundEmail(mx.exchange, 25, domainEmail, {
+            dkimDomain,
+            dkimSelector,
+          });
+
+          results.push({
+            domain,
+            success: true,
+            accepted: result.accepted,
+            rejected: result.rejected,
+          });
+          delivered = true;
+          logger.log('info', `MTA: delivered to ${domain} via ${mx.exchange}`);
+          break;
+        } catch (err: any) {
+          lastError = err.message;
+          logger.log('warn', `MTA: MX ${mx.exchange} failed for ${domain}: ${err.message}`);
+        }
      }

-      // In a full implementation, this would use the MTA service
-      // For now, we'll simulate a successful delivery
-      
-      logger.log('info', `Email processed by MTA: ${email.subject} to ${email.getAllRecipients().join(', ')}`);
-      
-      // Note: The MTA implementation would handle actual local delivery
-      
-      // Simulate successful delivery
-      return {
-        recipients: email.getAllRecipients().length,
-        subject: email.subject,
-        dkimSigned: !!item.route?.action.options?.mtaOptions?.dkimSign
-      };
-    } catch (error: any) {
-      logger.log('error', `Failed to process email in MTA mode: ${error.message}`);
-      throw error;
+      if (!delivered) {
+        results.push({ domain, success: false, error: lastError });
+        logger.log('error', `MTA: all MX hosts failed for ${domain}`);
+      }
    }
+
+    const allFailed = results.every(r => !r.success);
+    if (allFailed) {
+      const summary = results.map(r => `${r.domain}: ${r.error}`).join('; ');
+      throw new Error(`MTA delivery failed for all domains: ${summary}`);
+    }
+
+    return {
+      recipients: allRecipients.length,
+      domainResults: results,
+    };
  }
  
  /**
@@ -584,16 +666,10 @@ export class MultiModeDeliverySystem extends EventEmitter {
        await this.applyDkimSigning(email, item.route.action.options?.mtaOptions || {});
      }
      
-      logger.log('info', `Email successfully processed in store-and-forward mode`);
+      logger.log('info', `Email successfully processed in store-and-forward mode, delivering via MTA`);

-      // Simulate successful delivery
-      return {
-        recipients: email.getAllRecipients().length,
-        subject: email.subject,
-        scanned: !!route?.action.options?.contentScanning,
-        transformed: !!(route?.action.options?.transformations && route?.action.options?.transformations.length > 0),
-        dkimSigned: !!(item.route?.action.options?.mtaOptions?.dkimSign || item.route?.action.process?.dkim)
-      };
+      // After scanning + transformations, deliver via MTA
+      return await this.handleMtaDelivery(item);
    } catch (error: any) {
      logger.log('error', `Failed to process email: ${error.message}`);
      throw error;
--- a/ts/mail/routing/classes.dkim.manager.ts
+++ b/ts/mail/routing/classes.dkim.manager.ts
@@ -131,11 +131,15 @@ export class DkimManager {
      const { privateKey } = await this.dkimCreator.readDKIMKeys(domain);
      const rawEmail = email.toRFC822String();

+      // Detect key type from PEM header
+      const keyType = privateKey.includes('ED25519') ? 'ed25519' : 'rsa';
+
      const signResult = await this.rustBridge.signDkim({
        rawMessage: rawEmail,
        domain,
        selector,
        privateKey,
+        keyType,
      });

      if (signResult.header) {
--- a/ts/mail/routing/classes.email.action.executor.ts
+++ b/ts/mail/routing/classes.email.action.executor.ts
@@ -18,6 +18,7 @@ export interface IActionExecutorDeps {
    auth?: { user: string; pass: string };
    dkimDomain?: string;
    dkimSelector?: string;
+    tlsOpportunistic?: boolean;
  }) => Promise<ISmtpSendResult>;
  bounceManager: BounceManager;
  deliveryQueue: UnifiedDeliveryQueue;
--- a/ts/mail/routing/classes.unified.email.server.ts
+++ b/ts/mail/routing/classes.unified.email.server.ts
@@ -285,6 +285,7 @@ export class UnifiedEmailServer extends EventEmitter {
    auth?: { user: string; pass: string };
    dkimDomain?: string;
    dkimSelector?: string;
+    tlsOpportunistic?: boolean;
  }): Promise<ISmtpSendResult> {
    // Build DKIM config if domain has keys
    let dkim: { domain: string; selector: string; privateKey: string } | undefined;
@@ -321,6 +322,7 @@ export class UnifiedEmailServer extends EventEmitter {
      socketTimeoutSecs: Math.floor((this.options.outbound?.socketTimeout || 120000) / 1000),
      poolKey: `${host}:${port}`,
      maxPoolConnections: this.options.outbound?.maxConnections || 10,
+      tlsOpportunistic: options?.tlsOpportunistic ?? (port === 25),
    });
  }

@@ -416,6 +418,22 @@ export class UnifiedEmailServer extends EventEmitter {
        }
      }
    });
+
+    this.rustBridge.onScramCredentialRequest(async (data) => {
+      try {
+        await this.handleScramCredentialRequest(data);
+      } catch (err) {
+        logger.log('error', `Error handling SCRAM credential request: ${(err as Error).message}`);
+        try {
+          await this.rustBridge.sendScramCredentialResult({
+            correlationId: data.correlationId,
+            found: false,
+          });
+        } catch (sendErr) {
+          logger.log('warn', `Could not send SCRAM credential rejection: ${(sendErr as Error).message}`);
+        }
+      }
+    });
  }

  private async startSmtpServer(): Promise<void> {
@@ -622,6 +640,53 @@ export class UnifiedEmailServer extends EventEmitter {
    }
  }

+  /**
+   * Handle a SCRAM credential request from the Rust SMTP server.
+   * Computes SCRAM-SHA-256 credentials from the stored password for the given user.
+   */
+  private async handleScramCredentialRequest(data: { correlationId: string; username: string; remoteAddr: string }): Promise<void> {
+    const { correlationId, username, remoteAddr } = data;
+    const crypto = await import('crypto');
+
+    logger.log('info', `SCRAM credential request for user=${username} from=${remoteAddr}`);
+
+    const users = this.options.auth?.users || [];
+    const matched = users.find(u => u.username === username);
+
+    if (!matched) {
+      await this.rustBridge.sendScramCredentialResult({
+        correlationId,
+        found: false,
+      });
+      return;
+    }
+
+    // Compute SCRAM-SHA-256 credentials from plaintext password
+    const salt = crypto.randomBytes(16);
+    const iterations = 4096;
+
+    // SaltedPassword = PBKDF2-HMAC-SHA256(password, salt, iterations, 32)
+    const saltedPassword = crypto.pbkdf2Sync(matched.password, salt, iterations, 32, 'sha256');
+
+    // ClientKey = HMAC-SHA256(SaltedPassword, "Client Key")
+    const clientKey = crypto.createHmac('sha256', saltedPassword).update('Client Key').digest();
+
+    // StoredKey = SHA256(ClientKey)
+    const storedKey = crypto.createHash('sha256').update(clientKey).digest();
+
+    // ServerKey = HMAC-SHA256(SaltedPassword, "Server Key")
+    const serverKey = crypto.createHmac('sha256', saltedPassword).update('Server Key').digest();
+
+    await this.rustBridge.sendScramCredentialResult({
+      correlationId,
+      found: true,
+      salt: salt.toString('base64'),
+      iterations,
+      storedKey: storedKey.toString('base64'),
+      serverKey: serverKey.toString('base64'),
+    });
+  }
+
  /**
   * Verify inbound email security (DKIM/SPF/DMARC) using pre-computed Rust results
   * or falling back to IPC call if no pre-computed results are available.
--- a/ts/mail/security/classes.dkimcreator.ts
+++ b/ts/mail/security/classes.dkimcreator.ts
@@ -115,7 +115,7 @@ export class DKIMCreator {
    }
  }

-  // Create a DKIM key pair - changed to public for API access
+  // Create an RSA DKIM key pair - changed to public for API access
  public async createDKIMKeys(): Promise<{ privateKey: string; publicKey: string }> {
    const { privateKey, publicKey } = await generateKeyPair('rsa', {
      modulusLength: 2048,
@@ -126,6 +126,16 @@ export class DKIMCreator {
    return { privateKey, publicKey };
  }

+  // Create an Ed25519 DKIM key pair (RFC 8463)
+  public async createEd25519Keys(): Promise<{ privateKey: string; publicKey: string }> {
+    const { privateKey, publicKey } = await generateKeyPair('ed25519', {
+      publicKeyEncoding: { type: 'spki', format: 'pem' },
+      privateKeyEncoding: { type: 'pkcs8', format: 'pem' },
+    });
+
+    return { privateKey, publicKey };
+  }
+
  // Store a DKIM key pair - uses storage manager if available, else disk
  public async storeDKIMKeys(
    privateKey: string,
@@ -176,8 +186,11 @@ export class DKIMCreator {
      .replace(pemFooter, '')
      .replace(/\n/g, '');

+    // Detect key type from PEM header
+    const keyAlgo = keys.privateKey.includes('ED25519') || keys.publicKey.length < 200 ? 'ed25519' : 'rsa';
+
    // Now generate the DKIM DNS TXT record
-    const dnsRecordValue = `v=DKIM1; h=sha256; k=rsa; p=${keyContents}`;
+    const dnsRecordValue = `v=DKIM1; h=sha256; k=${keyAlgo}; p=${keyContents}`;

    return {
      name: `mta._domainkey.${domainArg}`,
@@ -375,8 +388,11 @@ export class DKIMCreator {
      .replace(pemFooter, '')
      .replace(/\n/g, '');

+    // Detect key type from PEM header
+    const keyAlgo = keys.privateKey.includes('ED25519') || keys.publicKey.length < 200 ? 'ed25519' : 'rsa';
+
    // Generate the DKIM DNS TXT record
-    const dnsRecordValue = `v=DKIM1; h=sha256; k=rsa; p=${keyContents}`;
+    const dnsRecordValue = `v=DKIM1; h=sha256; k=${keyAlgo}; p=${keyContents}`;

    return {
      name: `${selector}._domainkey.${domain}`,
--- a/ts/security/classes.rustsecuritybridge.ts
+++ b/ts/security/classes.rustsecuritybridge.ts
@@ -95,11 +95,12 @@ interface ISmtpSendOptions {
  domain?: string;
  auth?: { user: string; pass: string; method?: string };
  email: IOutboundEmail;
-  dkim?: { domain: string; selector: string; privateKey: string };
+  dkim?: { domain: string; selector: string; privateKey: string; keyType?: string };
  connectionTimeoutSecs?: number;
  socketTimeoutSecs?: number;
  poolKey?: string;
  maxPoolConnections?: number;
+  tlsOpportunistic?: boolean;
 }

 interface ISmtpSendRawOptions {
@@ -147,6 +148,7 @@ interface ISmtpServerConfig {
  securePort?: number;
  tlsCertPem?: string;
  tlsKeyPem?: string;
+  additionalTlsCerts?: Array<{ domains: string[]; certPem: string; keyPem: string }>;
  maxMessageSize?: number;
  maxConnections?: number;
  maxRecipients?: number;
@@ -193,6 +195,13 @@ interface IAuthRequestEvent {
  remoteAddr: string;
 }

+interface IScramCredentialRequestEvent {
+  correlationId: string;
+  sessionId: string;
+  username: string;
+  remoteAddr: string;
+}
+
 /**
 * Type-safe command map for the mailer-bin IPC bridge.
 */
@@ -222,7 +231,7 @@ type TMailerCommands = {
    result: IDkimVerificationResult[];
  };
  signDkim: {
-    params: { rawMessage: string; domain: string; selector?: string; privateKey: string };
+    params: { rawMessage: string; domain: string; selector?: string; privateKey: string; keyType?: string };
    result: { header: string; signedMessage: string };
  };
  checkSpf: {
@@ -273,6 +282,17 @@ type TMailerCommands = {
    };
    result: { resolved: boolean };
  };
+  scramCredentialResult: {
+    params: {
+      correlationId: string;
+      found: boolean;
+      salt?: string;
+      iterations?: number;
+      storedKey?: string;
+      serverKey?: string;
+    };
+    result: { resolved: boolean };
+  };
  configureRateLimits: {
    params: IRateLimitConfig;
    result: { configured: boolean };
@@ -706,12 +726,13 @@ export class RustSecurityBridge extends EventEmitter {
    return this.bridge.sendCommand('verifyDkim', { rawMessage });
  }

-  /** Sign an email with DKIM. */
+  /** Sign an email with DKIM (RSA or Ed25519). */
  public async signDkim(opts: {
    rawMessage: string;
    domain: string;
    selector?: string;
    privateKey: string;
+    keyType?: string;
  }): Promise<{ header: string; signedMessage: string }> {
    this.ensureRunning();
    return this.bridge.sendCommand('signDkim', opts);
@@ -829,6 +850,22 @@ export class RustSecurityBridge extends EventEmitter {
    await this.bridge.sendCommand('authResult', opts);
  }

+  /**
+   * Send SCRAM credentials back to the Rust SMTP server.
+   * Values (salt, storedKey, serverKey) must be base64-encoded.
+   */
+  public async sendScramCredentialResult(opts: {
+    correlationId: string;
+    found: boolean;
+    salt?: string;
+    iterations?: number;
+    storedKey?: string;
+    serverKey?: string;
+  }): Promise<void> {
+    this.ensureRunning();
+    await this.bridge.sendCommand('scramCredentialResult', opts);
+  }
+
  /** Update rate limit configuration at runtime. */
  public async configureRateLimits(config: IRateLimitConfig): Promise<void> {
    this.ensureRunning();
@@ -855,6 +892,14 @@ export class RustSecurityBridge extends EventEmitter {
    this.bridge.on('management:authRequest', handler);
  }

+  /**
+   * Register a handler for scramCredentialRequest events from the Rust SMTP server.
+   * The handler must call sendScramCredentialResult() with the correlationId.
+   */
+  public onScramCredentialRequest(handler: (data: IScramCredentialRequestEvent) => void): void {
+    this.bridge.on('management:scramCredentialRequest', handler);
+  }
+
  /** Remove an emailReceived event handler. */
  public offEmailReceived(handler: (data: IEmailReceivedEvent) => void): void {
    this.bridge.off('management:emailReceived', handler);
@@ -864,6 +909,11 @@ export class RustSecurityBridge extends EventEmitter {
  public offAuthRequest(handler: (data: IAuthRequestEvent) => void): void {
    this.bridge.off('management:authRequest', handler);
  }
+
+  /** Remove a scramCredentialRequest event handler. */
+  public offScramCredentialRequest(handler: (data: IScramCredentialRequestEvent) => void): void {
+    this.bridge.off('management:scramCredentialRequest', handler);
+  }
 }

 // Re-export interfaces for consumers
@@ -882,6 +932,7 @@ export type {
  IEmailData,
  IEmailReceivedEvent,
  IAuthRequestEvent,
+  IScramCredentialRequestEvent,
  IOutboundEmail,
  ISmtpSendResult,
  ISmtpSendOptions,
Author	SHA1	Message	Date
Juergen Kunz	9e722874b4	v5.1.2 Some checks failed CI / Type Check & Lint (push) Failing after 4s Details CI / Build Test (Current Platform) (push) Failing after 4s Details CI / Build All Platforms (push) Failing after 4s Details Publish to npm / npm-publish (push) Failing after 4s Details Release / build-and-release (push) Failing after 4s Details	2026-02-11 10:20:19 +00:00
Juergen Kunz	873af43ef2	fix(readme): adjust ASCII architecture diagram alignment in README	2026-02-11 10:20:19 +00:00
Juergen Kunz	76d898b648	v5.1.1 Some checks failed CI / Type Check & Lint (push) Failing after 4s Details CI / Build Test (Current Platform) (push) Failing after 4s Details CI / Build All Platforms (push) Failing after 4s Details Publish to npm / npm-publish (push) Failing after 5s Details Release / build-and-release (push) Failing after 4s Details	2026-02-11 10:16:30 +00:00
Juergen Kunz	b422639c34	fix(release): no changes	2026-02-11 10:16:30 +00:00
Juergen Kunz	c45ba2a7b4	v5.1.0 Some checks failed CI / Type Check & Lint (push) Failing after 4s Details CI / Build Test (Current Platform) (push) Failing after 4s Details CI / Build All Platforms (push) Failing after 7s Details Publish to npm / npm-publish (push) Failing after 6s Details Release / build-and-release (push) Failing after 4s Details	2026-02-11 10:11:43 +00:00
Juergen Kunz	b10597fd5e	feat(mailer-smtp): add SCRAM-SHA-256 auth, Ed25519 DKIM, opportunistic TLS, SNI cert selection, pipelining and delivery/bridge improvements	2026-02-11 10:11:43 +00:00