feat: Add comprehensive SMTP test suite for Deno

- Implemented SMTP client utilities in `test/helpers/smtp.client.ts` for creating test clients, sending emails, and testing connections.
- Developed SMTP protocol test utilities in `test/helpers/utils.ts` for managing TCP connections, sending commands, and handling responses.
- Created a detailed README in `test/readme.md` outlining the test framework, infrastructure, organization, and running instructions.
- Ported CMD-01: EHLO Command tests in `test/suite/smtpserver_commands/test.cmd-01.ehlo-command.test.ts` with multiple scenarios including valid and invalid hostnames.
- Ported CMD-02: MAIL FROM Command tests in `test/suite/smtpserver_commands/test.cmd-02.mail-from.test.ts` covering valid address acceptance, invalid address rejection, SIZE parameter support, and command sequence enforcement.

test/suite/smtpserver_commands/test.cmd-01.ehlo-command.test.ts

@@ -0,0 +1,154 @@
+/**
+ * CMD-01: EHLO Command Tests
+ * Tests SMTP EHLO command and server capabilities advertisement
+ */
+
+import { assert, assertEquals, assertMatch } from '@std/assert';
+import { startTestServer, stopTestServer, type ITestServer } from '../../helpers/server.loader.ts';
+import {
+  connectToSmtp,
+  waitForGreeting,
+  sendSmtpCommand,
+  closeSmtpConnection,
+} from '../../helpers/utils.ts';
+
+const TEST_PORT = 25251;
+let testServer: ITestServer;
+
+Deno.test({
+  name: 'CMD-01: Setup - Start SMTP server',
+  async fn() {
+    testServer = await startTestServer({ port: TEST_PORT });
+    assert(testServer, 'Test server should be created');
+  },
+  sanitizeResources: false,
+  sanitizeOps: false,
+});
+
+Deno.test({
+  name: 'CMD-01: EHLO Command - server responds with proper capabilities',
+  async fn() {
+    const conn = await connectToSmtp('localhost', TEST_PORT);
+
+    try {
+      // Wait for greeting
+      const greeting = await waitForGreeting(conn);
+      assert(greeting.includes('220'), 'Should receive 220 greeting');
+
+      // Send EHLO
+      const ehloResponse = await sendSmtpCommand(conn, 'EHLO test.example.com', '250');
+
+      // Parse capabilities
+      const lines = ehloResponse
+        .split('\r\n')
+        .filter((line) => line.startsWith('250'))
+        .filter((line) => line.length > 0);
+
+      const capabilities = lines.map((line) => line.substring(4).trim());
+      console.log('📋 Server capabilities:', capabilities);
+
+      // Verify essential capabilities
+      assert(
+        capabilities.some((cap) => cap.includes('SIZE')),
+        'Should advertise SIZE capability'
+      );
+      assert(
+        capabilities.some((cap) => cap.includes('8BITMIME')),
+        'Should advertise 8BITMIME capability'
+      );
+
+      // The last line should be "250 " (without hyphen)
+      const lastLine = lines[lines.length - 1];
+      assert(lastLine.startsWith('250 '), 'Last line should start with "250 " (space, not hyphen)');
+    } finally {
+      await closeSmtpConnection(conn);
+    }
+  },
+  sanitizeResources: false,
+  sanitizeOps: false,
+});
+
+Deno.test({
+  name: 'CMD-01: EHLO with invalid hostname - server handles gracefully',
+  async fn() {
+    const conn = await connectToSmtp('localhost', TEST_PORT);
+
+    try {
+      await waitForGreeting(conn);
+
+      const invalidHostnames = [
+        '',                    // Empty hostname
+        ' ',                   // Whitespace only
+        'invalid..hostname',   // Double dots
+        '.invalid',            // Leading dot
+        'invalid.',            // Trailing dot
+        'very-long-hostname-that-exceeds-reasonable-limits-' + 'x'.repeat(200),
+      ];
+
+      for (const hostname of invalidHostnames) {
+        console.log(`Testing invalid hostname: "${hostname}"`);
+
+        try {
+          const response = await sendSmtpCommand(conn, `EHLO ${hostname}`);
+          // Server should either accept with warning or reject with 5xx
+          assertMatch(response, /^(250|5\d\d)/, 'Server should respond with 250 or 5xx');
+
+          // Reset session for next test
+          if (response.startsWith('250')) {
+            await sendSmtpCommand(conn, 'RSET', '250');
+          }
+        } catch (error) {
+          // Some invalid hostnames might cause connection issues, which is acceptable
+          console.log(`  Hostname "${hostname}" caused error (acceptable):`, error.message);
+        }
+      }
+
+      // Send QUIT
+      await sendSmtpCommand(conn, 'QUIT', '221');
+    } finally {
+      try {
+        conn.close();
+      } catch {
+        // Ignore close errors
+      }
+    }
+  },
+  sanitizeResources: false,
+  sanitizeOps: false,
+});
+
+Deno.test({
+  name: 'CMD-01: EHLO command pipelining - multiple EHLO commands',
+  async fn() {
+    const conn = await connectToSmtp('localhost', TEST_PORT);
+
+    try {
+      await waitForGreeting(conn);
+
+      // First EHLO
+      const ehlo1Response = await sendSmtpCommand(conn, 'EHLO first.example.com', '250');
+      assert(ehlo1Response.startsWith('250'), 'First EHLO should succeed');
+
+      // Second EHLO (should reset session)
+      const ehlo2Response = await sendSmtpCommand(conn, 'EHLO second.example.com', '250');
+      assert(ehlo2Response.startsWith('250'), 'Second EHLO should succeed');
+
+      // Verify session was reset by trying MAIL FROM
+      const mailResponse = await sendSmtpCommand(conn, 'MAIL FROM:<test@example.com>', '250');
+      assert(mailResponse.startsWith('250'), 'MAIL FROM should work after second EHLO');
+    } finally {
+      await closeSmtpConnection(conn);
+    }
+  },
+  sanitizeResources: false,
+  sanitizeOps: false,
+});
+
+Deno.test({
+  name: 'CMD-01: Cleanup - Stop SMTP server',
+  async fn() {
+    await stopTestServer(testServer);
+  },
+  sanitizeResources: false,
+  sanitizeOps: false,
+});