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 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>
+        string CustomGreetingMessage { get; }
     }
 }

Src/SmtpServer/SmtpServerOptionsBuilder.cs

@@ -22,7 +22,8 @@ public ISmtpServerOptions Build()
                 MaxRetryCount = 5,
                 MaxAuthenticationAttempts = 3,
                 NetworkBufferSize = 128,
-                CommandWaitTimeout = TimeSpan.FromMinutes(5)
+                CommandWaitTimeout = TimeSpan.FromMinutes(5),
+                CustomGreetingMessage = null
             };
 
             _setters.ForEach(setter => setter(serverOptions));
@@ -155,6 +156,19 @@ 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="value">The greeting message to send to the client (e.g., "220 mail.example.com v1.0 ESMTP ready").</param>
+        /// <returns>An OptionsBuilder to continue building on.</returns>
+        public SmtpServerOptionsBuilder CustomGreetingMessage(string value)
+        {
+            _setters.Add(options => options.CustomGreetingMessage = value);
+
+            return this;
+        }
+
         #region SmtpServerOptions
 
         class SmtpServerOptions : ISmtpServerOptions
@@ -198,6 +212,13 @@ 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 string CustomGreetingMessage { 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;
@@ -187,9 +188,15 @@ 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($"220 {_context.ServerOptions.ServerName} v{version} ESMTP ready");
+            if (_context.ServerOptions.CustomGreetingMessage is null)
+            {
+                var serverVersion = AssemblyVersion;
+                _context.Pipe.Output.WriteLine($"220 {_context.ServerOptions.ServerName} v{serverVersion} ESMTP ready");
+            }
+            else
+            {
+                _context.Pipe.Output.WriteLine($"220 {_context.ServerOptions.ServerName} {_context.ServerOptions.CustomGreetingMessage}");
+            }
 
             return _context.Pipe.Output.FlushAsync(cancellationToken);
         }