Add support for custom SMTP greeting message (cosullivan#255)

* Add support for custom SMTP greeting message

Introduces a CustomGreetingMessage property to ISmtpServerOptions and SmtpServerOptionsBuilder, allowing configuration of the initial SMTP greeting sent to clients. SmtpSession now uses this custom message if set, otherwise defaults to the standard greeting with server name and version.

* optimize code

Author: tinohager

Src/SmtpServer/ISmtpServerOptions.cs

@@ -42,5 +42,12 @@ public interface ISmtpServerOptions
         /// The size of the buffer that is read from each call to the underlying network client.
         /// </summary>
         int NetworkBufferSize { get; }
+
+        /// <summary>
+        /// Gets the custom SMTP greeting message that the server sends immediately after a client connects,
+        /// typically as the initial "220" response. The message can be dynamically generated based on the session context.
+        /// If not set, a default greeting will be used (e.g., "220 mail.example.com ESMTP ready").
+        /// </summary>
+        Func<ISessionContext, string> CustomSmtpGreeting { get; }
     }
 }

Src/SmtpServer/SmtpServerOptionsBuilder.cs

@@ -24,6 +24,7 @@ public ISmtpServerOptions Build()
                 MaxAuthenticationAttempts = 3,
                 NetworkBufferSize = 128,
                 CommandWaitTimeout = TimeSpan.FromMinutes(5),
+                CustomSmtpGreeting = null,
             };
 
             _setters.ForEach(setter => setter(serverOptions));
@@ -157,6 +158,23 @@ public SmtpServerOptionsBuilder CommandWaitTimeout(TimeSpan value)
             return this;
         }
 
+        /// <summary>
+        /// Sets the custom SMTP greeting message sent to the client upon connection,
+        /// typically returned as the initial "220" response.
+        /// </summary>
+        /// <param name="smtpGreetingFunc">
+        /// A delegate that returns the greeting message to send to the client,
+        /// based on the <see cref="ISessionContext"/> (e.g., client IP, TLS state).
+        /// Example: <c>ctx => $"220 {sessionContext.ServerOptions.ServerName} ESMTP ready"</c>
+        /// </param>
+        /// <returns>An OptionsBuilder to continue building on.</returns>
+        public SmtpServerOptionsBuilder CustomGreetingMessage(Func<ISessionContext, string> smtpGreetingFunc)
+        {
+            _setters.Add(options => options.CustomSmtpGreeting = smtpGreetingFunc);
+
+            return this;
+        }
+
         #region SmtpServerOptions
 
         class SmtpServerOptions : ISmtpServerOptions
@@ -200,6 +218,13 @@ class SmtpServerOptions : ISmtpServerOptions
             /// The size of the buffer that is read from each call to the underlying network client.
             /// </summary>
             public int NetworkBufferSize { get; set; }
+
+            /// <summary>
+            /// Gets or sets the custom greeting message sent by the server in response to the initial SMTP connection.
+            /// This message is returned after the client connects and before any commands are issued (e.g., "220 mail.example.com v1.0 ESMTP ready").
+            /// If not set, a default greeting will be used.
+            /// </summary>
+            public Func<ISessionContext, string> CustomSmtpGreeting { get; set; }
         }
 
         #endregion

Src/SmtpServer/SmtpSession.cs

@@ -1,20 +1,21 @@
-using System;
-using System.Threading;
-using System.Threading.Tasks;
-using SmtpServer.Protocol;
-using System.Reflection;
+using SmtpServer.ComponentModel;
 using SmtpServer.IO;
-using System.IO.Pipelines;
+using SmtpServer.Protocol;
 using SmtpServer.StateMachine;
-using SmtpServer.ComponentModel;
+using System;
 using System.Buffers;
 using System.Collections.Generic;
+using System.IO.Pipelines;
+using System.Reflection;
+using System.Threading;
+using System.Threading.Tasks;
 
 namespace SmtpServer
 {
     internal sealed class SmtpSession
     {
         const string BufferKey = "SmtpSession:Buffer";
+        static readonly Version AssemblyVersion = typeof(SmtpSession).GetTypeInfo().Assembly.GetName().Version;
 
         readonly SmtpStateMachine _stateMachine;
         readonly SmtpSessionContext _context;
@@ -188,9 +189,15 @@ static async Task<bool> ExecuteAsync(SmtpCommand command, SmtpSessionContext con
         /// <returns>A task which performs the operation.</returns>
         ValueTask<FlushResult> OutputGreetingAsync(CancellationToken cancellationToken)
         {
-            var version = typeof(SmtpSession).GetTypeInfo().Assembly.GetName().Version;
-
-            _context.Pipe.Output.WriteLine($"220 {_context.ServerOptions.ServerName} v{version} ESMTP ready");
+            if (_context.ServerOptions.CustomSmtpGreeting is null)
+            {
+                var serverVersion = AssemblyVersion;
+                _context.Pipe.Output.WriteLine($"220 {_context.ServerOptions.ServerName} v{serverVersion} ESMTP ready");
+            }
+            else
+            {
+                _context.Pipe.Output.WriteLine(_context.ServerOptions.CustomSmtpGreeting(_context));
+            }
 
             return _context.Pipe.Output.FlushAsync(cancellationToken);
         }