Create a simple SMTP server in C#

roosrj

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20 Nov 2011CPOL

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A simple basic SMTP server for testing purposes

I created for testing purposes a simple SMTP server.

This is the client call:

System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("localhost");
smtp.Send(message);//Handles all messages in the protocol
smtp.Dispose();//sends a Quit message

This is the basic server code:

IPEndPoint endPoint = new IPEndPoint(IPAddress.Any, 25);
TcpListener listener = new TcpListener(endPoint);
listener.Start();

while (true)
{
    TcpClient client = listener.AcceptTcpClient();
    SMTPServer handler = new SMTPServer();
    servers.Add(handler);
    handler.Init(client);
    Thread thread = new System.Threading.Thread(new ThreadStart(handler.Run));
    thread.Start();
}

The protocol has the following messages in this example:

C : EHLO
S: 250 Ok

C : MAIL FROM
S: 250 OK

C : RCPT TO
S: 250 OK

C :DATA
S: 354 Start mail input; end with <crlf>.<crlf>

DATA....

S: 250 OK
C : Quit
</crlf></crlf>

Basically the Run method contains this:

public void Run()
{
    Write("220 localhost -- Fake proxy server");
    string strMessage = String.Empty;
    while (true)
    {
        try
        {
            strMessage = Read();
        }
        catch(Exception e)
        {
            //a socket error has occured
            break;
        }

        if (strMessage.Length > 0)
        {
            if (strMessage.StartsWith("QUIT"))
            {
                client.Close();
                break;//exit while
            }
            //message has successfully been received
            if (strMessage.StartsWith("EHLO"))
            {
                Write("250 OK");
            }

            if (strMessage.StartsWith("RCPT TO"))
            {
                Write("250 OK");
            }

            if (strMessage.StartsWith("MAIL FROM"))
            {

               Write("250 OK");
            }

            if (strMessage.StartsWith("DATA"))
            {
                Write("354 Start mail input; end with"); 
                strMessage = Read();
                Write("250 OK");
            }
        }  
    }
}

private void Write(String strMessage)
{
    NetworkStream clientStream = client.GetStream();
    ASCIIEncoding encoder = new ASCIIEncoding();
    byte[] buffer = encoder.GetBytes(strMessage + "\r\n");
    
    clientStream.Write(buffer, 0, buffer.Length);
    clientStream.Flush();
}

private String Read()
{
    byte[] messageBytes = new byte[8192];
    int bytesRead = 0;
    NetworkStream clientStream = client.GetStream();
    ASCIIEncoding encoder = new ASCIIEncoding();
    bytesRead = clientStream.Read(messageBytes, 0, 8192);
    string strMessage = encoder.GetString(messageBytes, 0, bytesRead);
    return strMessage;
}

Happy coding!

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By

roosrj

Netherlands

This member has not yet provided a Biography. Assume it's interesting and varied, and probably something to do with programming.

Comments and Discussions

First off, thanks for sharing the code. You definitely got me over the hump. I have implemented my own version of this based on your code. The problem I was having is that some mail clients use the same connection for multiple messages. Also, not all mail clients send a QUIT message. So I changed the code to not automatically close the connection on QUIT and also to handle a connection timing out.

The class is called MailListener (not claiming most accurate name here Smile | :)

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
using System.IO;

namespace FakeSmtp
{
    public class MailListener : TcpListener
    {
        private TcpClient client;
        private NetworkStream stream;
        private System.IO.StreamReader reader;
        private System.IO.StreamWriter writer;
        private Thread thread = null;

        public bool IsThreadAlive
        {
            get { return thread.IsAlive; }
        }

        public MailListener(IPAddress localaddr, int port)
            : base(localaddr, port)
        {
        }

        new public void Start()
        {
            base.Start();

            client = AcceptTcpClient();
            client.ReceiveTimeout = 5000;
            stream = client.GetStream();
            reader = new System.IO.StreamReader(stream);
            writer = new System.IO.StreamWriter(stream);
            writer.NewLine = "\r\n";
            writer.AutoFlush = true;

            thread = new System.Threading.Thread(new ThreadStart(RunThread));
            thread.Start();
        }

        protected void RunThread()
        {
            string line = null;

            writer.WriteLine("220 localhost -- Fake proxy server");

            try
            {
                while (reader != null)
                {
                    line = reader.ReadLine();
                    Console.Error.WriteLine("Read line {0}", line);

                    switch (line)
                    {
                        case "DATA":
                            writer.WriteLine("354 Start input, end data with <CRLF>.<CRLF>");
                            StringBuilder data = new StringBuilder();
                            String subject = "";
                            line = reader.ReadLine();

                            if (line != null && line != ".")
                            {
                                const string SUBJECT = "Subject: ";
                                if (line.StartsWith(SUBJECT))
                                {
                                    subject = line.Substring(SUBJECT.Length);
                                }
                                else
                                {
                                    data.AppendLine(line);
                                }

                                for (line = reader.ReadLine(); line != null && line != "."; line = reader.ReadLine())
                                {
                                    data.AppendLine(line);
                                }
                            }

                            String message = data.ToString();
                            Console.Error.WriteLine("Received  email with subject: {0} and message: {1}", subject, message);
                            writer.WriteLine("250 OK");
                            break;

                        case "QUIT":
                            writer.WriteLine("250 OK");
                            reader = null;
                            break;

                        default:
                            writer.WriteLine("250 OK");
                            break;
                    }
                }
            }
            catch (IOException)
            {
                Console.WriteLine("Connection lost.");
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex);
            }
            finally
            {
                client.Close();
                Stop();
            }
        }
    }
}

As you can see in the previous class, the thread management is now being handled by the MailListener class.

In a console application, the program.cs file would look like this.

using System;
using System.Text;
using System.Net;
using System.Net.Sockets;
using System.Threading;

namespace FakeSmtp
{
    public class SMTPServer
    {
        static void Main(string[] args)
        {
            MailListener listener = null;

            do
            {
                Console.WriteLine("New MailListener started");
                listener = new MailListener(IPAddress.Loopback, 25);
                listener.Start();
                while (listener.IsThreadAlive)
                {
                    Thread.Sleep(500);
                }
            } while (listener != null);
        }
    }
}

So there you have it. This is not the best way to architect this thing, but I was trying to spend as little time as possible to get the functionality I needed.

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.