Create a simple SMTP server in C#

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)

About the Author

roosrj

Netherlands

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	Profile popups Spacing Noise Layout Per page

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.