Add support for custom SMTP greeting message

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);
         }