Refactor SmtpSession to use predefined SmtpResponse constants

Replaces inline SmtpResponse instantiations in SmtpSession with static predefined response constants from SmtpResponse. Adds new static response properties for common SMTP replies related to TLS, authentication, and recipient/sender validation, improving code clarity and maintainability.

Author: Taiizor

src/Zetian/Internal/SmtpSession.cs

@@ -313,7 +313,7 @@ private async Task ProcessMailFromAsync(SmtpCommand command)
                 bool canAccept = await _configuration.MailboxFilter.CanAcceptFromAsync(this, mailFrom, messageSize).ConfigureAwait(false);
                 if (!canAccept)
                 {
-                    await SendResponseAsync(new SmtpResponse(550, "Sender rejected")).ConfigureAwait(false);
+                    await SendResponseAsync(SmtpResponse.SenderRejected).ConfigureAwait(false);
                     return;
                 }
             }
@@ -350,7 +350,7 @@ private async Task ProcessRcptToAsync(SmtpCommand command)
 
             if (_recipients.Count >= _configuration.MaxRecipients)
             {
-                await SendResponseAsync(new SmtpResponse(452, "Too many recipients")).ConfigureAwait(false);
+                await SendResponseAsync(SmtpResponse.TooManyRecipients).ConfigureAwait(false);
                 return;
             }
 
@@ -360,7 +360,7 @@ private async Task ProcessRcptToAsync(SmtpCommand command)
                 bool canDeliver = await _configuration.MailboxFilter.CanDeliverToAsync(this, rcptTo, _mailFrom ?? string.Empty).ConfigureAwait(false);
                 if (!canDeliver)
                 {
-                    await SendResponseAsync(new SmtpResponse(550, "Recipient rejected")).ConfigureAwait(false);
+                    await SendResponseAsync(SmtpResponse.RecipientRejected).ConfigureAwait(false);
                     return;
                 }
             }
@@ -517,7 +517,7 @@ private async Task ProcessStartTlsAsync(CancellationToken cancellationToken)
                 return;
             }
 
-            await SendResponseAsync(new SmtpResponse(220, "Ready to start TLS")).ConfigureAwait(false);
+            await SendResponseAsync(SmtpResponse.ReadyToStartTls).ConfigureAwait(false);
 
             try
             {
@@ -557,7 +557,7 @@ private async Task ProcessAuthAsync(SmtpCommand command, CancellationToken cance
 
             if (!IsSecure && !_configuration.AllowPlainTextAuthentication)
             {
-                await SendResponseAsync(new SmtpResponse(538, "Encryption required for authentication")).ConfigureAwait(false);
+                await SendResponseAsync(SmtpResponse.EncryptionRequiredForAuth).ConfigureAwait(false);
                 return;
             }
 
@@ -572,7 +572,7 @@ private async Task ProcessAuthAsync(SmtpCommand command, CancellationToken cance
 
             if (!_configuration.AuthenticationMechanisms.Contains(mechanism))
             {
-                await SendResponseAsync(new SmtpResponse(504, "Authentication mechanism not supported")).ConfigureAwait(false);
+                await SendResponseAsync(SmtpResponse.AuthMechanismNotSupported).ConfigureAwait(false);
                 return;
             }
 
@@ -590,7 +590,7 @@ private async Task ProcessAuthAsync(SmtpCommand command, CancellationToken cance
             {
                 IsAuthenticated = true;
                 AuthenticatedIdentity = result.Identity;
-                await SendResponseAsync(new SmtpResponse(235, "Authentication successful")).ConfigureAwait(false);
+                await SendResponseAsync(SmtpResponse.AuthenticationSuccessful).ConfigureAwait(false);
             }
             else
             {

src/Zetian/Protocol/SmtpResponse.cs

@@ -248,6 +248,31 @@ private static string GetDefaultMessage(int code)
         /// <summary>554 Transaction failed</summary>
         public static readonly SmtpResponse TransactionFailed = new(554, "Transaction failed");
 
+        #region TLS and Authentication Responses
+
+        /// <summary>220 Ready to start TLS</summary>
+        public static readonly SmtpResponse ReadyToStartTls = new(220, "Ready to start TLS");
+
+        /// <summary>235 Authentication successful</summary>
+        public static readonly SmtpResponse AuthenticationSuccessful = new(235, "Authentication successful");
+
+        /// <summary>452 Too many recipients</summary>
+        public static readonly SmtpResponse TooManyRecipients = new(452, "Too many recipients");
+
+        /// <summary>504 Authentication mechanism not supported</summary>
+        public static readonly SmtpResponse AuthMechanismNotSupported = new(504, "Authentication mechanism not supported");
+
+        /// <summary>538 Encryption required for authentication</summary>
+        public static readonly SmtpResponse EncryptionRequiredForAuth = new(538, "Encryption required for authentication");
+
+        /// <summary>550 Sender rejected</summary>
+        public static readonly SmtpResponse SenderRejected = new(550, "Sender rejected");
+
+        /// <summary>550 Recipient rejected</summary>
+        public static readonly SmtpResponse RecipientRejected = new(550, "Recipient rejected");
+
+        #endregion
+
         #endregion
     }
 }