dcrouter/ts/mail/delivery/classes.smtp.client.legacy.ts

import * as plugins from '../../plugins.js';
import { logger } from '../../logger.js';
import { 
  SecurityLogger, 
  SecurityLogLevel, 
  SecurityEventType 
} from '../../security/index.js';

import { 
  MtaConnectionError, 
  MtaAuthenticationError,
  MtaDeliveryError,
  MtaConfigurationError,
  MtaTimeoutError,
  MtaProtocolError
} from '../../errors/index.js';

import { Email } from '../core/classes.email.js';
import type { EmailProcessingMode } from './interfaces.js';

// Custom error type extension
interface NodeNetworkError extends Error {
  code?: string;
}

/**
 * SMTP client connection options
 */
export type ISmtpClientOptions = {
  /**
   * Hostname of the SMTP server
   */
  host: string;
  
  /**
   * Port to connect to
   */
  port: number;
  
  /**
   * Whether to use TLS for the connection
   */
  secure?: boolean;
  
  /**
   * Connection timeout in milliseconds
   */
  connectionTimeout?: number;
  
  /**
   * Socket timeout in milliseconds
   */
  socketTimeout?: number;
  
  /**
   * Command timeout in milliseconds
   */
  commandTimeout?: number;
  
  /**
   * TLS options
   */
  tls?: {
    /**
     * Whether to verify certificates
     */
    rejectUnauthorized?: boolean;
    
    /**
     * Minimum TLS version
     */
    minVersion?: string;
    
    /**
     * CA certificate path
     */
    ca?: string;
  };
  
  /**
   * Authentication options
   */
  auth?: {
    /**
     * Authentication user
     */
    user: string;
    
    /**
     * Authentication password
     */
    pass: string;
    
    /**
     * Authentication method
     */
    method?: 'PLAIN' | 'LOGIN' | 'OAUTH2';
  };
  
  /**
   * Domain name for EHLO
   */
  domain?: string;
  
  /**
   * DKIM options for signing outgoing emails
   */
  dkim?: {
    /**
     * Whether to sign emails with DKIM
     */
    enabled: boolean;
    
    /**
     * Domain name for DKIM
     */
    domain: string;
    
    /**
     * Selector for DKIM
     */
    selector: string;
    
    /**
     * Private key for DKIM signing
     */
    privateKey: string;
    
    /**
     * Headers to sign
     */
    headers?: string[];
  };
};

/**
 * SMTP delivery result
 */
export type ISmtpDeliveryResult = {
  /**
   * Whether the delivery was successful
   */
  success: boolean;
  
  /**
   * Message ID if successful
   */
  messageId?: string;
  
  /**
   * Error message if failed
   */
  error?: string;
  
  /**
   * SMTP response code
   */
  responseCode?: string;
  
  /**
   * Recipients successfully delivered to
   */
  acceptedRecipients: string[];
  
  /**
   * Recipients rejected during delivery
   */
  rejectedRecipients: string[];
  
  /**
   * Server response
   */
  response?: string;
  
  /**
   * Timestamp of the delivery attempt
   */
  timestamp: number;
  
  /**
   * Whether DKIM signing was applied
   */
  dkimSigned?: boolean;
  
  /**
   * Whether this was a TLS secured delivery
   */
  secure?: boolean;
  
  /**
   * Whether authentication was used
   */
  authenticated?: boolean;
};

/**
 * SMTP client for sending emails to remote mail servers
 */
export class SmtpClient {
  private options: ISmtpClientOptions;
  private connected: boolean = false;
  private socket?: plugins.net.Socket | plugins.tls.TLSSocket;
  private supportedExtensions: Set<string> = new Set();
  
  /**
   * Create a new SMTP client instance
   * @param options SMTP client connection options
   */
  constructor(options: ISmtpClientOptions) {
    // Set default options
    this.options = {
      ...options,
      connectionTimeout: options.connectionTimeout || 30000, // 30 seconds
      socketTimeout: options.socketTimeout || 60000, // 60 seconds
      commandTimeout: options.commandTimeout || 30000, // 30 seconds
      secure: options.secure || false,
      domain: options.domain || 'localhost',
      tls: {
        rejectUnauthorized: options.tls?.rejectUnauthorized !== false, // Default to true
        minVersion: options.tls?.minVersion || 'TLSv1.2'
      }
    };
  }
  
  /**
   * Connect to the SMTP server
   */
  public async connect(): Promise<void> {
    if (this.connected && this.socket) {
      return;
    }
    
    try {
      logger.log('info', `Connecting to SMTP server ${this.options.host}:${this.options.port}`);
      
      // Create socket
      const socket = new plugins.net.Socket();
      
      // Set timeouts
      socket.setTimeout(this.options.socketTimeout);
      
      // Connect to the server
      await new Promise<void>((resolve, reject) => {
        // Handle connection events
        socket.once('connect', () => {
          logger.log('debug', `Connected to ${this.options.host}:${this.options.port}`);
          resolve();
        });
        
        socket.once('timeout', () => {
          reject(MtaConnectionError.timeout(
            this.options.host,
            this.options.port,
            this.options.connectionTimeout
          ));
        });
        
        socket.once('error', (err: NodeNetworkError) => {
          if (err.code === 'ECONNREFUSED') {
            reject(MtaConnectionError.refused(
              this.options.host,
              this.options.port
            ));
          } else if (err.code === 'ENOTFOUND') {
            reject(MtaConnectionError.dnsError(
              this.options.host,
              err
            ));
          } else {
            reject(new MtaConnectionError(
              `Connection error to ${this.options.host}:${this.options.port}: ${err.message}`,
              {
                data: {
                  host: this.options.host,
                  port: this.options.port,
                  error: err.message,
                  code: err.code
                }
              }
            ));
          }
        });
        
        // Connect to the server
        const connectOptions = {
          host: this.options.host,
          port: this.options.port
        };
        
        // For direct TLS connections
        if (this.options.secure) {
          const tlsSocket = plugins.tls.connect({
            ...connectOptions,
            rejectUnauthorized: this.options.tls.rejectUnauthorized,
            minVersion: this.options.tls.minVersion as any,
            ca: this.options.tls.ca ? [this.options.tls.ca] : undefined
          } as plugins.tls.ConnectionOptions);
          
          tlsSocket.once('secureConnect', () => {
            logger.log('debug', `Secure connection established to ${this.options.host}:${this.options.port}`);
            this.socket = tlsSocket;
            resolve();
          });
          
          tlsSocket.once('error', (err: NodeNetworkError) => {
            reject(new MtaConnectionError(
              `TLS connection error to ${this.options.host}:${this.options.port}: ${err.message}`,
              {
                data: {
                  host: this.options.host,
                  port: this.options.port,
                  error: err.message,
                  code: err.code
                }
              }
            ));
          });
          
          tlsSocket.setTimeout(this.options.socketTimeout);
          
          tlsSocket.once('timeout', () => {
            reject(MtaConnectionError.timeout(
              this.options.host,
              this.options.port,
              this.options.connectionTimeout
            ));
          });
        } else {
          socket.connect(connectOptions);
          this.socket = socket;
        }
      });
      
      // Wait for server greeting
      const greeting = await this.readResponse();
      
      if (!greeting.startsWith('220')) {
        throw new MtaConnectionError(
          `Unexpected greeting from server: ${greeting}`,
          {
            data: {
              host: this.options.host,
              port: this.options.port,
              greeting
            }
          }
        );
      }
      
      // Send EHLO
      await this.sendEhlo();
      
      // Start TLS if not secure and supported
      if (!this.options.secure && this.supportedExtensions.has('STARTTLS')) {
        await this.startTls();
        
        // Send EHLO again after STARTTLS
        await this.sendEhlo();
      }
      
      // Authenticate if credentials provided
      if (this.options.auth) {
        await this.authenticate();
      }
      
      this.connected = true;
      logger.log('info', `Successfully connected to SMTP server ${this.options.host}:${this.options.port}`);
      
      // Set up error handling for the socket
      this.socket.on('error', (err) => {
        logger.log('error', `Socket error: ${err.message}`);
        this.connected = false;
        this.socket = undefined;
      });
      
      this.socket.on('close', () => {
        logger.log('debug', 'Socket closed');
        this.connected = false;
        this.socket = undefined;
      });
      
      this.socket.on('timeout', () => {
        logger.log('error', 'Socket timeout');
        this.connected = false;
        if (this.socket) {
          this.socket.destroy();
          this.socket = undefined;
        }
      });
      
    } catch (error) {
      // Clean up socket if connection failed
      if (this.socket) {
        this.socket.destroy();
        this.socket = undefined;
      }
      
      logger.log('error', `Failed to connect to SMTP server: ${error.message}`);
      throw error;
    }
  }
  
  /**
   * Send EHLO command to the server
   */
  private async sendEhlo(): Promise<void> {
    // Clear previous extensions
    this.supportedExtensions.clear();
    
    // Send EHLO - don't allow pipelining for this command
    const response = await this.sendCommand(`EHLO ${this.options.domain}`, false);
    
    // Parse supported extensions
    const lines = response.split('\r\n');
    for (let i = 1; i < lines.length; i++) {
      const line = lines[i];
      if (line.startsWith('250-') || line.startsWith('250 ')) {
        const extension = line.substring(4).split(' ')[0];
        this.supportedExtensions.add(extension);
      }
    }
    
    // Check if server supports pipelining
    this.supportsPipelining = this.supportedExtensions.has('PIPELINING');
    
    logger.log('debug', `Server supports extensions: ${Array.from(this.supportedExtensions).join(', ')}`);
    if (this.supportsPipelining) {
      logger.log('info', 'Server supports PIPELINING - will use for improved performance');
    }
  }
  
  /**
   * Start TLS negotiation
   */
  private async startTls(): Promise<void> {
    logger.log('debug', 'Starting TLS negotiation');
    
    // Send STARTTLS command
    const response = await this.sendCommand('STARTTLS');
    
    if (!response.startsWith('220')) {
      throw new MtaConnectionError(
        `Failed to start TLS: ${response}`,
        {
          data: {
            host: this.options.host,
            port: this.options.port,
            response
          }
        }
      );
    }
    
    if (!this.socket) {
      throw new MtaConnectionError(
        'No socket available for TLS upgrade',
        {
          data: {
            host: this.options.host,
            port: this.options.port
          }
        }
      );
    }
    
    // Upgrade socket to TLS
    const currentSocket = this.socket;
    this.socket = await this.upgradeTls(currentSocket);
  }
  
  /**
   * Upgrade socket to TLS
   * @param socket Original socket
   */
  private async upgradeTls(socket: plugins.net.Socket): Promise<plugins.tls.TLSSocket> {
    return new Promise<plugins.tls.TLSSocket>((resolve, reject) => {
      const tlsOptions: plugins.tls.ConnectionOptions = {
        socket,
        servername: this.options.host,
        rejectUnauthorized: this.options.tls.rejectUnauthorized,
        minVersion: this.options.tls.minVersion as any,
        ca: this.options.tls.ca ? [this.options.tls.ca] : undefined
      };
      
      const tlsSocket = plugins.tls.connect(tlsOptions);
      
      tlsSocket.once('secureConnect', () => {
        logger.log('debug', 'TLS negotiation successful');
        resolve(tlsSocket);
      });
      
      tlsSocket.once('error', (err: NodeNetworkError) => {
        reject(new MtaConnectionError(
          `TLS error: ${err.message}`,
          {
            data: {
              host: this.options.host,
              port: this.options.port,
              error: err.message,
              code: err.code
            }
          }
        ));
      });
      
      tlsSocket.setTimeout(this.options.socketTimeout);
      
      tlsSocket.once('timeout', () => {
        reject(MtaTimeoutError.commandTimeout(
          'STARTTLS',
          this.options.host,
          this.options.socketTimeout
        ));
      });
    });
  }
  
  /**
   * Authenticate with the server
   */
  private async authenticate(): Promise<void> {
    if (!this.options.auth) {
      return;
    }
    
    const { user, pass, method = 'LOGIN' } = this.options.auth;
    
    logger.log('debug', `Authenticating as ${user} using ${method}`);
    
    try {
      switch (method) {
        case 'PLAIN':
          await this.authPlain(user, pass);
          break;
          
        case 'LOGIN':
          await this.authLogin(user, pass);
          break;
          
        case 'OAUTH2':
          await this.authOAuth2(user, pass);
          break;
          
        default:
          throw new MtaAuthenticationError(
            `Authentication method ${method} not supported by client`,
            {
              data: {
                method
              }
            }
          );
      }
      
      logger.log('info', `Successfully authenticated as ${user}`);
    } catch (error) {
      logger.log('error', `Authentication failed: ${error.message}`);
      throw error;
    }
  }
  
  /**
   * Authenticate using PLAIN method
   * @param user Username
   * @param pass Password
   */
  private async authPlain(user: string, pass: string): Promise<void> {
    // PLAIN authentication format: \0username\0password
    const authString = Buffer.from(`\0${user}\0${pass}`).toString('base64');
    const response = await this.sendCommand(`AUTH PLAIN ${authString}`);
    
    if (!response.startsWith('235')) {
      throw MtaAuthenticationError.invalidCredentials(
        this.options.host,
        user
      );
    }
  }
  
  /**
   * Authenticate using LOGIN method
   * @param user Username
   * @param pass Password
   */
  private async authLogin(user: string, pass: string): Promise<void> {
    // Start LOGIN authentication
    const response = await this.sendCommand('AUTH LOGIN');
    
    if (!response.startsWith('334')) {
      throw new MtaAuthenticationError(
        `Server did not accept AUTH LOGIN: ${response}`,
        {
          data: {
            host: this.options.host,
            response
          }
        }
      );
    }
    
    // Send username (base64)
    const userResponse = await this.sendCommand(Buffer.from(user).toString('base64'));
    
    if (!userResponse.startsWith('334')) {
      throw MtaAuthenticationError.invalidCredentials(
        this.options.host,
        user
      );
    }
    
    // Send password (base64)
    const passResponse = await this.sendCommand(Buffer.from(pass).toString('base64'));
    
    if (!passResponse.startsWith('235')) {
      throw MtaAuthenticationError.invalidCredentials(
        this.options.host,
        user
      );
    }
  }
  
  /**
   * Authenticate using OAuth2 method
   * @param user Username
   * @param token OAuth2 token
   */
  private async authOAuth2(user: string, token: string): Promise<void> {
    // XOAUTH2 format
    const authString = `user=${user}\x01auth=Bearer ${token}\x01\x01`;
    const response = await this.sendCommand(`AUTH XOAUTH2 ${Buffer.from(authString).toString('base64')}`);
    
    if (!response.startsWith('235')) {
      throw MtaAuthenticationError.invalidCredentials(
        this.options.host,
        user
      );
    }
  }
  
  /**
   * Send an email through the SMTP client
   * @param email Email to send
   * @param processingMode Optional processing mode
   */
  public async sendMail(email: Email, processingMode?: EmailProcessingMode): Promise<ISmtpDeliveryResult> {
    // Ensure we're connected
    if (!this.connected || !this.socket) {
      await this.connect();
    }
    
    const startTime = Date.now();
    const result: ISmtpDeliveryResult = {
      success: false,
      acceptedRecipients: [],
      rejectedRecipients: [],
      timestamp: startTime,
      secure: this.options.secure || this.socket instanceof plugins.tls.TLSSocket,
      authenticated: !!this.options.auth
    };
    
    try {
      logger.log('info', `Sending email to ${email.getAllRecipients().join(', ')}`);
      
      // Apply DKIM signing if configured
      if (this.options.dkim?.enabled) {
        await this.applyDkimSignature(email);
        result.dkimSigned = true;
      }
      
      // Get envelope and recipients
      const envelope_from = email.getEnvelopeFrom() || email.from;
      const recipients = email.getAllRecipients();
      
      // Check if we can use pipelining for MAIL FROM and RCPT TO commands
      if (this.supportsPipelining && recipients.length > 0) {
        logger.log('debug', 'Using SMTP pipelining for sending');
        
        // Send MAIL FROM command first (always needed)
        const mailFromCmd = `MAIL FROM:<${envelope_from}> SIZE=${this.getEmailSize(email)}`;
        let mailFromResponse: string;
        
        try {
          mailFromResponse = await this.sendCommand(mailFromCmd);
          
          if (!mailFromResponse.startsWith('250')) {
            throw new MtaDeliveryError(
              `MAIL FROM command failed: ${mailFromResponse}`,
              {
                data: {
                  command: mailFromCmd,
                  response: mailFromResponse
                }
              }
            );
          }
        } catch (error) {
          logger.log('error', `MAIL FROM failed: ${error.message}`);
          throw error;
        }
        
        // Pipeline all RCPT TO commands
        const rcptPromises = recipients.map(recipient => {
          return this.sendCommand(`RCPT TO:<${recipient}>`)
            .then(response => {
              if (response.startsWith('250')) {
                result.acceptedRecipients.push(recipient);
                return { recipient, accepted: true, response };
              } else {
                result.rejectedRecipients.push(recipient);
                logger.log('warn', `Recipient ${recipient} rejected: ${response}`);
                return { recipient, accepted: false, response };
              }
            })
            .catch(error => {
              result.rejectedRecipients.push(recipient);
              logger.log('warn', `Recipient ${recipient} rejected with error: ${error.message}`);
              return { recipient, accepted: false, error: error.message };
            });
        });
        
        // Wait for all RCPT TO commands to complete
        await Promise.all(rcptPromises);
      } else {
        // Fall back to sequential commands if pipelining not supported
        logger.log('debug', 'Using sequential SMTP commands for sending');
        
        // Send MAIL FROM
        await this.sendCommand(`MAIL FROM:<${envelope_from}> SIZE=${this.getEmailSize(email)}`);
        
        // Send RCPT TO for each recipient
        for (const recipient of recipients) {
          try {
            await this.sendCommand(`RCPT TO:<${recipient}>`);
            result.acceptedRecipients.push(recipient);
          } catch (error) {
            logger.log('warn', `Recipient ${recipient} rejected: ${error.message}`);
            result.rejectedRecipients.push(recipient);
          }
        }
      }
      
      // Check if at least one recipient was accepted
      if (result.acceptedRecipients.length === 0) {
        throw new MtaDeliveryError(
          'All recipients were rejected',
          {
            data: {
              recipients,
              rejectedRecipients: result.rejectedRecipients
            }
          }
        );
      }
      
      // Send DATA
      const dataResponse = await this.sendCommand('DATA');
      
      if (!dataResponse.startsWith('354')) {
        throw new MtaProtocolError(
          `Failed to start DATA phase: ${dataResponse}`,
          {
            data: {
              response: dataResponse
            }
          }
        );
      }
      
      // Format email content efficiently
      const emailContent = await this.getFormattedEmail(email);
      
      // Send email content
      const finalResponse = await this.sendCommand(emailContent + '\r\n.');
      
      // Extract message ID if available
      const messageIdMatch = finalResponse.match(/\[(.*?)\]/);
      if (messageIdMatch) {
        result.messageId = messageIdMatch[1];
      }
      
      result.success = true;
      result.response = finalResponse;
      
      logger.log('info', `Email sent successfully to ${result.acceptedRecipients.join(', ')}`);
      
      // Log security event
      SecurityLogger.getInstance().logEvent({
        level: SecurityLogLevel.INFO,
        type: SecurityEventType.EMAIL_DELIVERY,
        message: 'Email sent successfully',
        details: {
          recipients: result.acceptedRecipients,
          rejectedRecipients: result.rejectedRecipients,
          messageId: result.messageId,
          secure: result.secure,
          authenticated: result.authenticated,
          server: `${this.options.host}:${this.options.port}`,
          dkimSigned: result.dkimSigned
        },
        success: true
      });
      
      return result;
    } catch (error) {
      logger.log('error', `Failed to send email: ${error.message}`);
      
      // Format error for result
      result.error = error.message;
      
      // Extract SMTP code if available
      if (error.context?.data?.statusCode) {
        result.responseCode = error.context.data.statusCode;
      }
      
      // Log security event
      SecurityLogger.getInstance().logEvent({
        level: SecurityLogLevel.ERROR,
        type: SecurityEventType.EMAIL_DELIVERY,
        message: 'Email delivery failed',
        details: {
          error: error.message,
          server: `${this.options.host}:${this.options.port}`,
          recipients: email.getAllRecipients(),
          acceptedRecipients: result.acceptedRecipients,
          rejectedRecipients: result.rejectedRecipients,
          secure: result.secure,
          authenticated: result.authenticated
        },
        success: false
      });
      
      return result;
    }
  }
  
  /**
   * Apply DKIM signature to email
   * @param email Email to sign
   */
  private async applyDkimSignature(email: Email): Promise<void> {
    if (!this.options.dkim?.enabled || !this.options.dkim?.privateKey) {
      return;
    }
    
    try {
      logger.log('debug', `Signing email with DKIM for domain ${this.options.dkim.domain}`);
      
      // Format email for DKIM signing
      const { dkimSign } = plugins;
      const emailContent = await this.getFormattedEmail(email);
      
      // Sign email
      const signOptions = {
        domainName: this.options.dkim.domain,
        keySelector: this.options.dkim.selector,
        privateKey: this.options.dkim.privateKey,
        headerFieldNames: this.options.dkim.headers || [
          'from', 'to', 'subject', 'date', 'message-id'
        ]
      };
      
      const signedEmail = await dkimSign(emailContent, signOptions);
      
      // Replace headers in original email
      const dkimHeader = signedEmail.substring(0, signedEmail.indexOf('\r\n\r\n')).split('\r\n')
        .find(line => line.startsWith('DKIM-Signature: '));
      
      if (dkimHeader) {
        email.addHeader('DKIM-Signature', dkimHeader.substring('DKIM-Signature: '.length));
      }
      
      logger.log('debug', 'DKIM signature applied successfully');
    } catch (error) {
      logger.log('error', `Failed to apply DKIM signature: ${error.message}`);
      throw error;
    }
  }
  
  /**
   * Format email for SMTP transmission
   * @param email Email to format
   */
  private async getFormattedEmail(email: Email): Promise<string> {
    // This is a simplified implementation
    // In a full implementation, this would use proper MIME formatting
    
    let content = '';
    
    // Add headers
    content += `From: ${email.from}\r\n`;
    content += `To: ${email.to.join(', ')}\r\n`;
    content += `Subject: ${email.subject}\r\n`;
    content += `Date: ${new Date().toUTCString()}\r\n`;
    content += `Message-ID: <${plugins.uuid.v4()}@${this.options.domain}>\r\n`;
    
    // Add additional headers
    for (const [name, value] of Object.entries(email.headers || {})) {
      content += `${name}: ${value}\r\n`;
    }
    
    // Add content type for multipart
    if (email.attachments && email.attachments.length > 0) {
      const boundary = `----_=_NextPart_${Math.random().toString(36).substr(2)}`;
      content += `MIME-Version: 1.0\r\n`;
      content += `Content-Type: multipart/mixed; boundary="${boundary}"\r\n`;
      content += `\r\n`;
      
      // Add text part
      content += `--${boundary}\r\n`;
      content += `Content-Type: text/plain; charset="UTF-8"\r\n`;
      content += `\r\n`;
      content += `${email.text}\r\n`;
      
      // Add HTML part if present
      if (email.html) {
        content += `--${boundary}\r\n`;
        content += `Content-Type: text/html; charset="UTF-8"\r\n`;
        content += `\r\n`;
        content += `${email.html}\r\n`;
      }
      
      // Add attachments
      for (const attachment of email.attachments) {
        content += `--${boundary}\r\n`;
        content += `Content-Type: ${attachment.contentType || 'application/octet-stream'}; name="${attachment.filename}"\r\n`;
        content += `Content-Disposition: attachment; filename="${attachment.filename}"\r\n`;
        content += `Content-Transfer-Encoding: base64\r\n`;
        content += `\r\n`;
        
        // Add base64 encoded content
        const base64Content = attachment.content.toString('base64');
        
        // Split into lines of 76 characters
        for (let i = 0; i < base64Content.length; i += 76) {
          content += base64Content.substring(i, i + 76) + '\r\n';
        }
      }
      
      // End boundary
      content += `--${boundary}--\r\n`;
    } else {
      // Simple email with just text
      content += `Content-Type: text/plain; charset="UTF-8"\r\n`;
      content += `\r\n`;
      content += `${email.text}\r\n`;
    }
    
    return content;
  }
  
  /**
   * Get size of email in bytes
   * @param email Email to measure
   */
  private getEmailSize(email: Email): number {
    // Simplified size estimation
    let size = 0;
    
    // Headers
    size += `From: ${email.from}\r\n`.length;
    size += `To: ${email.to.join(', ')}\r\n`.length;
    size += `Subject: ${email.subject}\r\n`.length;
    
    // Body
    size += (email.text?.length || 0) + 2; // +2 for CRLF
    
    // HTML part if present
    if (email.html) {
      size += email.html.length + 2;
    }
    
    // Attachments
    for (const attachment of email.attachments || []) {
      size += attachment.content.length;
    }
    
    // Add overhead for MIME boundaries and headers
    const overhead = email.attachments?.length ? 1000 + (email.attachments.length * 200) : 200;
    
    return size + overhead;
  }
  
  /**
   * Send SMTP command and wait for response
   * @param command SMTP command to send
   */
  // Queue for command pipelining
  private commandQueue: Array<{
    command: string;
    resolve: (response: string) => void;
    reject: (error: any) => void;
    timeout: NodeJS.Timeout;
  }> = [];
  
  // Flag to indicate if we're currently processing commands
  private processingCommands = false;
  
  // Flag to indicate if server supports pipelining
  private supportsPipelining = false;
  
  /**
   * Send an SMTP command and wait for response
   * @param command SMTP command to send
   * @param allowPipelining Whether this command can be pipelined
   */
  private async sendCommand(command: string, allowPipelining = true): Promise<string> {
    if (!this.socket) {
      throw new MtaConnectionError(
        'Not connected to server',
        {
          data: {
            host: this.options.host,
            port: this.options.port
          }
        }
      );
    }
    
    // Log command if not sensitive
    if (!command.startsWith('AUTH')) {
      logger.log('debug', `> ${command}`);
    } else {
      logger.log('debug', '> AUTH ***');
    }
    
    return new Promise<string>((resolve, reject) => {
      // Set up timeout for command
      const timeout = setTimeout(() => {
        // Remove this command from the queue if it times out
        const index = this.commandQueue.findIndex(item => item.command === command);
        if (index !== -1) {
          this.commandQueue.splice(index, 1);
        }
        
        reject(MtaTimeoutError.commandTimeout(
          command.split(' ')[0],
          this.options.host,
          this.options.commandTimeout
        ));
      }, this.options.commandTimeout);
      
      // Add command to the queue
      this.commandQueue.push({
        command,
        resolve,
        reject,
        timeout
      });
      
      // Process command queue if we can pipeline or if not currently processing commands
      if ((this.supportsPipelining && allowPipelining) || !this.processingCommands) {
        this.processCommandQueue();
      }
    });
  }
  
  /**
   * Process the command queue - either one by one or pipelined if supported
   */
  private processCommandQueue(): void {
    if (this.processingCommands || this.commandQueue.length === 0 || !this.socket) {
      return;
    }
    
    this.processingCommands = true;
    
    try {
      // If pipelining is supported, send all commands at once
      if (this.supportsPipelining) {
        // Send all commands in queue at once
        const commands = this.commandQueue.map(item => item.command).join('\r\n') + '\r\n';
        
        this.socket.write(commands, (err) => {
          if (err) {
            // Handle write error for all commands
            const error = new MtaConnectionError(
              `Failed to send commands: ${err.message}`,
              {
                data: {
                  error: err.message
                }
              }
            );
            
            // Fail all pending commands
            while (this.commandQueue.length > 0) {
              const item = this.commandQueue.shift();
              clearTimeout(item.timeout);
              item.reject(error);
            }
            
            this.processingCommands = false;
          }
        });
        
        // Process responses one by one in order
        this.processResponses();
      } else {
        // Process commands one by one if pipelining not supported
        this.processNextCommand();
      }
    } catch (error) {
      logger.log('error', `Error processing command queue: ${error.message}`);
      this.processingCommands = false;
    }
  }
  
  /**
   * Process the next command in the queue (non-pipelined mode)
   */
  private processNextCommand(): void {
    if (this.commandQueue.length === 0 || !this.socket) {
      this.processingCommands = false;
      return;
    }
    
    const currentCommand = this.commandQueue[0];
    
    this.socket.write(currentCommand.command + '\r\n', (err) => {
      if (err) {
        // Handle write error
        const error = new MtaConnectionError(
          `Failed to send command: ${err.message}`,
          {
            data: {
              command: currentCommand.command.split(' ')[0],
              error: err.message
            }
          }
        );
        
        // Remove from queue
        this.commandQueue.shift();
        clearTimeout(currentCommand.timeout);
        currentCommand.reject(error);
        
        // Continue with next command
        this.processNextCommand();
        return;
      }
      
      // Read response
      this.readResponse()
        .then((response) => {
          // Remove from queue and resolve
          this.commandQueue.shift();
          clearTimeout(currentCommand.timeout);
          currentCommand.resolve(response);
          
          // Process next command
          this.processNextCommand();
        })
        .catch((err) => {
          // Remove from queue and reject
          this.commandQueue.shift();
          clearTimeout(currentCommand.timeout);
          currentCommand.reject(err);
          
          // Process next command
          this.processNextCommand();
        });
    });
  }
  
  /**
   * Process responses for pipelined commands
   */
  private async processResponses(): Promise<void> {
    try {
      // Process responses for each command in order
      while (this.commandQueue.length > 0) {
        const currentCommand = this.commandQueue[0];
        
        try {
          // Wait for response
          const response = await this.readResponse();
          
          // Remove from queue and resolve
          this.commandQueue.shift();
          clearTimeout(currentCommand.timeout);
          currentCommand.resolve(response);
        } catch (error) {
          // Remove from queue and reject
          this.commandQueue.shift();
          clearTimeout(currentCommand.timeout);
          currentCommand.reject(error);
          
          // Stop processing if this is a critical error
          if (
            error instanceof MtaConnectionError &&
            (error.message.includes('Connection closed') || error.message.includes('Not connected'))
          ) {
            break;
          }
        }
      }
    } catch (error) {
      logger.log('error', `Error processing responses: ${error.message}`);
    } finally {
      this.processingCommands = false;
    }
  }
  
  /**
   * Read response from the server
   */
  private async readResponse(): Promise<string> {
    if (!this.socket) {
      throw new MtaConnectionError(
        'Not connected to server',
        {
          data: {
            host: this.options.host,
            port: this.options.port
          }
        }
      );
    }
    
    return new Promise<string>((resolve, reject) => {
      // Use an array to collect response chunks instead of string concatenation
      const responseChunks: Buffer[] = [];
      
      // Single function to clean up all listeners
      const cleanupListeners = () => {
        if (!this.socket) return;
        this.socket.removeListener('data', onData);
        this.socket.removeListener('error', onError);
        this.socket.removeListener('close', onClose);
        this.socket.removeListener('end', onEnd);
      };
      
      const onData = (data: Buffer) => {
        // Store buffer directly, avoiding unnecessary string conversion
        responseChunks.push(data);
        
        // Convert to string only for response checking
        const responseData = Buffer.concat(responseChunks).toString();
        
        // Check if this is a complete response
        if (this.isCompleteResponse(responseData)) {
          // Clean up listeners
          cleanupListeners();
          
          const trimmedResponse = responseData.trim();
          logger.log('debug', `< ${trimmedResponse}`);
          
          // Check if this is an error response
          if (this.isErrorResponse(responseData)) {
            const code = responseData.substring(0, 3);
            reject(this.createErrorFromResponse(trimmedResponse, code));
          } else {
            resolve(trimmedResponse);
          }
        }
      };
      
      const onError = (err: Error) => {
        cleanupListeners();
        
        reject(new MtaConnectionError(
          `Socket error while waiting for response: ${err.message}`,
          {
            data: {
              error: err.message
            }
          }
        ));
      };
      
      const onClose = () => {
        cleanupListeners();
        
        const responseData = Buffer.concat(responseChunks).toString();
        reject(new MtaConnectionError(
          'Connection closed while waiting for response',
          {
            data: {
              partialResponse: responseData
            }
          }
        ));
      };
      
      const onEnd = () => {
        cleanupListeners();
        
        const responseData = Buffer.concat(responseChunks).toString();
        reject(new MtaConnectionError(
          'Connection ended while waiting for response',
          {
            data: {
              partialResponse: responseData
            }
          }
        ));
      };
      
      // Set up listeners
      this.socket.on('data', onData);
      this.socket.once('error', onError);
      this.socket.once('close', onClose);
      this.socket.once('end', onEnd);
    });
  }
  
  /**
   * Check if the response is complete
   * @param response Response to check
   */
  private isCompleteResponse(response: string): boolean {
    // Check if it's a multi-line response
    const lines = response.split('\r\n');
    const lastLine = lines[lines.length - 2]; // Second to last because of the trailing CRLF
    
    // Check if the last line starts with a code followed by a space
    // If it does, this is a complete response
    if (lastLine && /^\d{3} /.test(lastLine)) {
      return true;
    }
    
    // For single line responses
    if (lines.length === 2 && lines[0].length >= 3 && /^\d{3} /.test(lines[0])) {
      return true;
    }
    
    return false;
  }
  
  /**
   * Check if the response is an error
   * @param response Response to check
   */
  private isErrorResponse(response: string): boolean {
    // Get the status code (first 3 characters)
    const code = response.substring(0, 3);
    
    // 4xx and 5xx are error codes
    return code.startsWith('4') || code.startsWith('5');
  }
  
  /**
   * Create appropriate error from response
   * @param response Error response
   * @param code SMTP status code
   */
  private createErrorFromResponse(response: string, code: string): Error {
    // Extract message part
    const message = response.substring(4).trim();
    
    switch (code.charAt(0)) {
      case '4': // Temporary errors
        return MtaDeliveryError.temporary(
          message,
          'recipient',
          code,
          response
        );
        
      case '5': // Permanent errors
        return MtaDeliveryError.permanent(
          message,
          'recipient',
          code,
          response
        );
        
      default:
        return new MtaDeliveryError(
          `Unexpected error response: ${response}`,
          {
            data: {
              response,
              code
            }
          }
        );
    }
  }
  
  /**
   * Close the connection to the server
   */
  public async close(): Promise<void> {
    if (!this.connected || !this.socket) {
      return;
    }
    
    try {
      // Send QUIT
      await this.sendCommand('QUIT');
    } catch (error) {
      logger.log('warn', `Error sending QUIT command: ${error.message}`);
    } finally {
      // Close socket
      this.socket.destroy();
      this.socket = undefined;
      this.connected = false;
      logger.log('info', 'SMTP connection closed');
    }
  }
  
  /**
   * Checks if the connection is active
   */
  public isConnected(): boolean {
    return this.connected && !!this.socket;
  }
  
  /**
   * Update SMTP client options
   * @param options New options
   */
  public updateOptions(options: Partial<ISmtpClientOptions>): void {
    this.options = {
      ...this.options,
      ...options
    };
    
    logger.log('info', 'SMTP client options updated');
  }
}