fix: resolved all missing enhanced status codes from this.send usage and added fallback for 2xx, 4xx, and 5xx

Author: titanism

lib/smtp-connection.js

@@ -22,12 +22,15 @@ const ENHANCED_STATUS_CODES = {
     214: '2.0.0', // Help message
     220: '2.0.0', // Service ready
     221: '2.0.0', // Service closing transmission channel
+    235: '2.7.0', // Authentication successful
     250: '2.0.0', // Requested mail action okay, completed
     251: '2.1.5', // User not local; will forward
     252: '2.1.5', // Cannot VRFY user, but will accept message
+    334: '3.7.0', // Server challenge for authentication
     354: '2.0.0', // Start mail input; end with <CRLF>.<CRLF>
 
     // Temporary failure codes (4xx)
+    420: '4.4.2', // Timeout or connection lost (non-standard, used by some servers)
     421: '4.4.2', // Service not available, closing transmission channel
     450: '4.2.1', // Requested mail action not taken: mailbox unavailable
     451: '4.3.0', // Requested action aborted: local error in processing
@@ -41,14 +44,19 @@ const ENHANCED_STATUS_CODES = {
     503: '5.5.1', // Bad sequence of commands
     504: '5.5.4', // Command parameter not implemented
     521: '5.3.2', // Machine does not accept mail
+    523: '5.3.4', // Message size exceeds server limit (non-standard, used by some servers)
     530: '5.7.0', // Authentication required
     535: '5.7.8', // Authentication credentials invalid
+    538: '5.7.0', // Must issue a STARTTLS command first (non-standard)
     550: '5.1.1', // Requested action not taken: mailbox unavailable
     551: '5.1.6', // User not local; please try forwarding
     552: '5.2.2', // Requested mail action aborted: exceeded storage allocation
     553: '5.1.3', // Requested action not taken: mailbox name not allowed
     554: '5.6.0', // Transaction failed
-    556: '5.1.10' // RCPT TO syntax error
+    555: '5.5.4', // MAIL FROM/RCPT TO parameters not recognized or not implemented
+    556: '5.1.10', // RCPT TO syntax error (non-standard)
+    557: '5.7.1', // Delivery not authorized (non-standard, used by some servers)
+    558: '5.2.3' // Message too large for recipient (non-standard, used by some servers)
 };
 
 // Skip enhanced status codes for initial greeting and HELO/EHLO responses
@@ -674,7 +682,27 @@ class SMTPConnection extends EventEmitter {
         }
 
         // Use default mapping
-        return ENHANCED_STATUS_CODES[code] || '';
+        if (ENHANCED_STATUS_CODES[code]) {
+            return ENHANCED_STATUS_CODES[code];
+        }
+
+        // 2xx fallback
+        if (code >= 200 && code < 300) {
+            return '2.0.0';
+        }
+
+        // 4xx (transient failure)
+        if (code >= 400 && code < 500) {
+            return '4.0.0';
+        }
+
+        // 5xx (permanent failure)
+        if (code >= 500) {
+            return '5.0.0';
+        }
+
+        // safeguard (non-spec; but should never occur)
+        return '';
     }
 
     /**