Add configurable max message size enforcement

Src/SmtpServer.Tests/PipeReaderTests.cs

@@ -24,8 +24,10 @@
             // arrange
             var reader = CreatePipeReader("abcde\r\n");
 
+            var maxMessageSizeOptions = new MaxMessageSizeOptions();
+
             // act
-            var line = await reader.ReadLineAsync(Encoding.ASCII).ConfigureAwait(false);
+            var line = await reader.ReadLineAsync(Encoding.ASCII, maxMessageSizeOptions).ConfigureAwait(false);
 
             // assert
             Assert.Equal(5, line.Length);
@@ -39,8 +41,10 @@
             // arrange
             var reader = CreatePipeReader("ab\rcd\ne\r\n");
 
+            var maxMessageSizeOptions = new MaxMessageSizeOptions();
+
             // act
-            var line = await reader.ReadLineAsync(Encoding.ASCII).ConfigureAwait(false);
+            var line = await reader.ReadLineAsync(Encoding.ASCII, maxMessageSizeOptions).ConfigureAwait(false);
 
             // assert
             Assert.Equal(7, line.Length);
@@ -54,10 +58,12 @@
             // arrange
             var reader = CreatePipeReader("abcde\r\nfghij\r\nklmno\r\n");
 
+            var maxMessageSizeOptions = new MaxMessageSizeOptions();
+
             // act
-            var line1 = await reader.ReadLineAsync(Encoding.ASCII).ConfigureAwait(false);
-            var line2 = await reader.ReadLineAsync(Encoding.ASCII).ConfigureAwait(false);
-            var line3 = await reader.ReadLineAsync(Encoding.ASCII).ConfigureAwait(false);
+            var line1 = await reader.ReadLineAsync(Encoding.ASCII, maxMessageSizeOptions).ConfigureAwait(false);
+            var line2 = await reader.ReadLineAsync(Encoding.ASCII, maxMessageSizeOptions).ConfigureAwait(false);
+            var line3 = await reader.ReadLineAsync(Encoding.ASCII, maxMessageSizeOptions).ConfigureAwait(false);
 
             // assert
             Assert.Equal("abcde", line1);
@@ -71,6 +77,8 @@
             // arrange
             var reader = CreatePipeReader("abcd\r\n..1234\r\n.\r\n");
 
+            var maxMessageSizeOptions = new MaxMessageSizeOptions();
+
             // act
             var text = "";
             await reader.ReadDotBlockAsync(
@@ -79,7 +87,8 @@
                     text = StringUtil.Create(buffer);
 
                     return Task.CompletedTask;
-                });
+                },
+                maxMessageSizeOptions);
 
             // assert
             Assert.Equal("abcd\r\n.1234", text);

Src/SmtpServer.Tests/SmtpServerTests.cs

@@ -1,4 +1,5 @@
 using MailKit;
+using MailKit.Net.Smtp;
 using SmtpServer.Authentication;
 using SmtpServer.ComponentModel;
 using SmtpServer.Mail;
@@ -9,6 +10,7 @@
 using System;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 using System.Net;
 using System.Net.Security;
 using System.Net.Sockets;
@@ -45,7 +47,7 @@
                 // assert
                 Assert.Single(MessageStore.Messages);
                 Assert.Equal("[email protected]", MessageStore.Messages[0].Transaction.From.AsAddress());
                 Assert.Equal(1, MessageStore.Messages[0].Transaction.To.Count);
                 Assert.Equal("[email protected]", MessageStore.Messages[0].Transaction.To[0].AsAddress());
             }
         }
@@ -152,16 +154,30 @@
 
                 for (var i = 0; i < 5; i++)
                 {
                     Task.Delay(TimeSpan.FromMilliseconds(250)).Wait();
                     client.NoOp();
                 }
 
                 Task.Delay(TimeSpan.FromSeconds(5)).Wait();
 
                 Assert.Throws<IOException>(() => client.NoOp());
             }
         }
 
+        [Fact]
+        public void WillTerminateDueToTooMuchData()
+        {
+            var maxAcceptedMailMessageSize = 50;
+
+            var largeMailContent = string.Concat(Enumerable.Repeat("Too long for 1024 bytes", 1000));
+            using var mailMessage = MailClient.Message(from: "[email protected]", to: "[email protected]", text: largeMailContent);
+
+            using (CreateServer(c => c.MaxMessageSize(maxAcceptedMailMessageSize, MaxMessageSizeHandling.Strict)))
+            {
+                Assert.Throws<SmtpCommandException>(() => MailClient.Send(mailMessage));
+            }
+        }
+
         [Fact]
         public async Task WillSessionTimeoutDuringMailDataTransmission()
         {

Src/SmtpServer/IMaxMessageSizeOptions.cs

@@ -0,0 +1,19 @@
+namespace SmtpServer
+{
+    /// <summary>
+    /// Defines configuration options for enforcing a maximum allowed message size according to the SMTP SIZE extension (RFC 1870).
+    /// Includes the size limit in bytes and the handling strategy for oversized messages.
+    /// </summary>
+    public interface IMaxMessageSizeOptions
+    {
+        /// <summary>
+        /// Gets or sets the maximum allowed message size in bytes.
+        /// </summary>
+        int Length { get; }
+
+        /// <summary>
+        /// Gets the handling type an oversized message.
+        /// </summary>
+        MaxMessageSizeHandling Handling { get; }
+    }
+}

Src/SmtpServer/IO/PipeReaderExtensions.cs

@@ -4,6 +4,7 @@
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
+using SmtpServer.Protocol;
 using SmtpServer.Text;
 
 namespace SmtpServer.IO
@@ -21,9 +22,10 @@ internal static class PipeReaderExtensions
         /// <param name="reader">The reader to read from.</param>
         /// <param name="sequence">The sequence to find to terminate the read operation.</param>
         /// <param name="func">The callback to execute to process the buffer.</param>
+        /// <param name="maxMessageSizeOptions">Handling of MaxMessageSize.</param>
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>The value that was read from the buffer.</returns>
-        static async ValueTask ReadUntilAsync(PipeReader reader, byte[] sequence, Func<ReadOnlySequence<byte>, Task> func, CancellationToken cancellationToken)
+        static async ValueTask ReadUntilAsync(PipeReader reader, byte[] sequence, Func<ReadOnlySequence<byte>, Task> func, IMaxMessageSizeOptions maxMessageSizeOptions, CancellationToken cancellationToken)
         {
             if (reader == null)
             {
@@ -35,6 +37,11 @@ static async ValueTask ReadUntilAsync(PipeReader reader, byte[] sequence, Func<R
 
             while (read.IsCanceled == false && read.IsCompleted == false && read.Buffer.IsEmpty == false)
             {
+                if (maxMessageSizeOptions.Handling == MaxMessageSizeHandling.Strict && read.Buffer.Length > maxMessageSizeOptions.Length)
+                {
+                    throw new SmtpResponseException(SmtpResponse.MaxMessageSizeExceeded, true);
+                }
+
                 if (read.Buffer.TryFind(sequence, ref head, out var tail))
                 {
                     try
@@ -60,42 +67,45 @@ static async ValueTask ReadUntilAsync(PipeReader reader, byte[] sequence, Func<R
         /// </summary>
         /// <param name="reader">The reader to read from.</param>
         /// <param name="func">The action to process the buffer.</param>
+        /// <param name="maxMessageSizeOptions">Handling of MaxMessageSize.</param>
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>A task that can be used to wait on the operation on complete.</returns>
-        internal static ValueTask ReadLineAsync(this PipeReader reader, Func<ReadOnlySequence<byte>, Task> func, CancellationToken cancellationToken = default)
+        internal static ValueTask ReadLineAsync(this PipeReader reader, Func<ReadOnlySequence<byte>, Task> func, IMaxMessageSizeOptions maxMessageSizeOptions, CancellationToken cancellationToken = default)
         {
             if (reader == null)
             {
                 throw new ArgumentNullException(nameof(reader));
             }
 
-            return ReadUntilAsync(reader, CRLF, func, cancellationToken);
+            return ReadUntilAsync(reader, CRLF, func, maxMessageSizeOptions, cancellationToken);
         }
 
         /// <summary>
         /// Reads a line from the reader.
         /// </summary>
         /// <param name="reader">The reader to read from.</param>
+        /// <param name="maxMessageSizeOptions">Handling of MaxMessageSize.</param>
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>A task that can be used to wait on the operation on complete.</returns>
-        internal static ValueTask<string> ReadLineAsync(this PipeReader reader, CancellationToken cancellationToken = default)
+        internal static ValueTask<string> ReadLineAsync(this PipeReader reader, IMaxMessageSizeOptions maxMessageSizeOptions, CancellationToken cancellationToken = default)
         {
             if (reader == null)
             {
                 throw new ArgumentNullException(nameof(reader));
             }
 
-            return reader.ReadLineAsync(Encoding.ASCII, cancellationToken);
+            return reader.ReadLineAsync(Encoding.ASCII, maxMessageSizeOptions, cancellationToken);
         }
 
         /// <summary>
         /// Reads a line from the reader.
         /// </summary>
         /// <param name="reader">The reader to read from.</param>
         /// <param name="encoding">The encoding to use when converting the input.</param>
+        /// <param name="maxMessageSizeOptions"> Handling of MaxMessageSize</param>
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>A task that can be used to wait on the operation on complete.</returns>
-        internal static async ValueTask<string> ReadLineAsync(this PipeReader reader, Encoding encoding, CancellationToken cancellationToken = default)
+        internal static async ValueTask<string> ReadLineAsync(this PipeReader reader, Encoding encoding, IMaxMessageSizeOptions maxMessageSizeOptions, CancellationToken cancellationToken = default)
         {
             if (reader == null)
             {
@@ -111,6 +121,7 @@ await reader.ReadLineAsync(
 
                     return Task.CompletedTask;
                 },
+                maxMessageSizeOptions,
                 cancellationToken);
 
             return text;
@@ -121,24 +132,26 @@ await reader.ReadLineAsync(
         /// </summary>
         /// <param name="reader">The reader to read from.</param>
         /// <param name="func">The action to process the buffer.</param>
+        /// <param name="maxMessageSizeOptions">Handling of MaxMessageSize.</param>
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>The value that was read from the buffer.</returns>
-        internal static async ValueTask ReadDotBlockAsync(this PipeReader reader, Func<ReadOnlySequence<byte>, Task> func, CancellationToken cancellationToken = default)
+        internal static async ValueTask ReadDotBlockAsync(this PipeReader reader, Func<ReadOnlySequence<byte>, Task> func, IMaxMessageSizeOptions maxMessageSizeOptions, CancellationToken cancellationToken = default)
         {
             if (reader == null)
             {
                 throw new ArgumentNullException(nameof(reader));
             }
 
             await ReadUntilAsync(
-                reader, 
-                DotBlock, 
+                reader,
+                DotBlock,
                 buffer =>
                 {
                     buffer = Unstuff(buffer);
 
                     return func(buffer);
-                }, 
+                },
+                maxMessageSizeOptions,
                 cancellationToken);
 
             static ReadOnlySequence<byte> Unstuff(ReadOnlySequence<byte> buffer)

Src/SmtpServer/ISmtpServerOptions.cs

@@ -9,9 +9,9 @@ namespace SmtpServer
     public interface ISmtpServerOptions
     {
         /// <summary>
-        /// Gets the maximum size of a message.
+        /// Gets the maximum message size option.
         /// </summary>
-        int MaxMessageSize { get; }
+        IMaxMessageSizeOptions MaxMessageSizeOptions { get; }
 
         /// <summary>
         /// The maximum number of retries before quitting the session.

Src/SmtpServer/MaxMessageSizeHandling.cs

@@ -0,0 +1,18 @@
+namespace SmtpServer
+{
+    /// <summary>
+    /// Choose how MaxMessageSize limit should be considered
+    /// </summary>
+    public enum MaxMessageSizeHandling
+    {
+        /// <summary>
+        /// Use the size limit for the SIZE extension of ESMTP
+        /// </summary>
+        Ignore = 0,
+
+        /// <summary>
+        /// Close the session after too much data has been sent
+        /// </summary>
+        Strict = 1,
+    }
+}

Src/SmtpServer/MaxMessageSizeOptions.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace SmtpServer
+{
+    /// <summary>
+    /// Represents configuration settings for enforcing a maximum message size in SMTP,
+    /// including the size limit in bytes and the behavior when that limit is exceeded.
+    /// </summary>
+    public class MaxMessageSizeOptions : IMaxMessageSizeOptions
+    {
+        /// <summary>
+        /// Gets or sets the maximum allowed message size in bytes,
+        /// as specified by the SMTP SIZE extension (RFC 1870).
+        /// </summary>
+        public int Length { get; set; }
+
+        /// <summary>
+        /// Gets or sets the handling strategy for messages that exceed the maximum allowed size.
+        /// </summary>
+        public MaxMessageSizeHandling Handling { get; set; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MaxMessageSizeOptions"/> class 
+        /// with the specified handling strategy and message size limit.
+        /// </summary>
+        /// <param name="handling">The strategy for handling messages that exceed the maximum allowed size.</param>
+        /// <param name="length">The maximum allowed message size in bytes.</param>
+        public MaxMessageSizeOptions(MaxMessageSizeHandling handling, int length)
+        {
+            Length = length;
+            Handling = handling;
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MaxMessageSizeOptions"/> class with default values.
+        /// </summary>
+        public MaxMessageSizeOptions()
+        {
+
+        }
+    }
+}

Src/SmtpServer/Protocol/AuthCommand.cs

@@ -106,7 +106,7 @@
             {
                 await context.Pipe.Output.WriteReplyAsync(new SmtpResponse(SmtpReplyCode.ContinueWithAuth, " "), cancellationToken).ConfigureAwait(false);
 
-                authentication = await context.Pipe.Input.ReadLineAsync(Encoding.ASCII, cancellationToken).ConfigureAwait(false);
+                authentication = await context.Pipe.Input.ReadLineAsync(Encoding.ASCII, context.ServerOptions.MaxMessageSizeOptions, cancellationToken).ConfigureAwait(false);
             }
 
             if (TryExtractFromBase64(authentication) == false)
@@ -155,13 +155,13 @@
                 //Username = VXNlcm5hbWU6 (base64)
                 await context.Pipe.Output.WriteReplyAsync(new SmtpResponse(SmtpReplyCode.ContinueWithAuth, "VXNlcm5hbWU6"), cancellationToken).ConfigureAwait(false);
 
-                _user = await ReadBase64EncodedLineAsync(context.Pipe.Input, cancellationToken).ConfigureAwait(false);
+                _user = await ReadBase64EncodedLineAsync(context.Pipe.Input, context.ServerOptions.MaxMessageSizeOptions, cancellationToken).ConfigureAwait(false);
             }
 
             //Password = UGFzc3dvcmQ6 (base64)
             await context.Pipe.Output.WriteReplyAsync(new SmtpResponse(SmtpReplyCode.ContinueWithAuth, "UGFzc3dvcmQ6"), cancellationToken).ConfigureAwait(false);
 
-            _password = await ReadBase64EncodedLineAsync(context.Pipe.Input, cancellationToken).ConfigureAwait(false);
+            _password = await ReadBase64EncodedLineAsync(context.Pipe.Input, context.ServerOptions.MaxMessageSizeOptions, cancellationToken).ConfigureAwait(false);
 
             return true;
         }
@@ -172,9 +172,9 @@
         /// <param name="reader">The pipe to read from.</param>
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>The decoded Base64 string.</returns>
-        static async Task<string> ReadBase64EncodedLineAsync(PipeReader reader, CancellationToken cancellationToken)
+        static async Task<string> ReadBase64EncodedLineAsync(PipeReader reader, IMaxMessageSizeOptions maxMessageSizeOptions, CancellationToken cancellationToken)
         {
-            var text = await reader.ReadLineAsync(cancellationToken);
+            var text = await reader.ReadLineAsync(maxMessageSizeOptions, cancellationToken);
 
             return text == null ? string.Empty : Encoding.UTF8.GetString(Convert.FromBase64String(text));
         }

Src/SmtpServer/Protocol/DataCommand.cs

@@ -52,7 +52,8 @@ await context.Pipe.Input.ReadDotBlockAsync(
                     {
                         // ReSharper disable once AccessToDisposedClosure
                         response = await container.Instance.SaveAsync(context, context.Transaction, buffer, cancellationToken).ConfigureAwait(false);
-                    }, 
+                    },
+                    context.ServerOptions.MaxMessageSizeOptions,
                     cancellationToken).ConfigureAwait(false);
 
                 await context.Pipe.Output.WriteReplyAsync(response, cancellationToken).ConfigureAwait(false);

Src/SmtpServer/Protocol/EhloCommand.cs

@@ -75,9 +75,9 @@ protected virtual IEnumerable<string> GetExtensions(ISessionContext context)
                 yield return "STARTTLS";
             }
 
-            if (context.ServerOptions.MaxMessageSize > 0)
+            if (context.ServerOptions.MaxMessageSizeOptions.Length > 0)
             {
-                yield return $"SIZE {context.ServerOptions.MaxMessageSize}";
+                yield return $"SIZE {context.ServerOptions.MaxMessageSizeOptions.Length}";
             }
 
             if (IsPlainLoginAllowed(context))

Src/SmtpServer/Protocol/MailCommand.cs

@@ -51,7 +51,7 @@ internal override async Task<bool> ExecuteAsync(SmtpSessionContext context, Canc
             var size = GetMessageSize();
 
             // check against the server supplied maximum
-            if (context.ServerOptions.MaxMessageSize > 0 && size > context.ServerOptions.MaxMessageSize)
+            if (context.ServerOptions.MaxMessageSizeOptions.Length > 0 && size > context.ServerOptions.MaxMessageSizeOptions.Length)
             {
                 await context.Pipe.Output.WriteReplyAsync(SmtpResponse.SizeLimitExceeded, cancellationToken).ConfigureAwait(false);
                 return false;

Src/SmtpServer/Protocol/SmtpResponse.cs

@@ -70,6 +70,11 @@ public class SmtpResponse
         /// </summary>
         public static readonly SmtpResponse AuthenticationRequired = new SmtpResponse(SmtpReplyCode.AuthenticationRequired, "authentication required");
 
+        /// <summary>
+        /// 552 MaxMessageSizeExceeded
+        /// </summary>
+        public static readonly SmtpResponse MaxMessageSizeExceeded = new SmtpResponse(SmtpReplyCode.SizeLimitExceeded, "message size exceeds fixed maximium message size");
+
         /// <summary>
         /// Constructor.
         /// </summary>