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Create a simple SMTP server in C#

By , 20 Nov 2011
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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 Netherlands
No Biography provided

Comments and Discussions

 
SuggestionSome suggestion to improve your tip PinmemberObiWan_MVP3-Sep-12 6:13 
First of all, I think that you left out too much stuff from your example and this, in turn, makes it difficult to understand; it would have been a better idea adding some "full" code like the one shown here[^]; then, it would be useful to add some code to the "Run()" method to set the client socket "ReceiveTimeout" property (see here[^]) to a value different from "infinite" (aka zero); for example 5000, this way, a client connecting to the server and idling withouth sending in any command for some time (five seconds in my example) would be automatically disconnected.
 
just to be clear, here's the modified code snippet from the first URL above
 
using System;
using System.Text;
using System.Net;
using System.Net.Sockets;
using System.Threading;
 
namespace FakeSMTP
{
    public class SMTPServer
    {
        TcpClient client;
        NetworkStream stream;
        System.IO.StreamReader reader;
        System.IO.StreamWriter writer;
        
        public SMTPServer(TcpClient client)
        {
            this.client = client;
            this.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;
        }
 
        static void Main(string[] args)
        {
            TcpListener listener = new TcpListener(IPAddress.Loopback,25);
            listener.Start();
            while (true)
            {
                SMTPServer handler = new SMTPServer(listener.AcceptTcpClient());
                Thread thread = new System.Threading.Thread(new ThreadStart(handler.Run));
                thread.Start();
            }
        }
        
        public void Run()
        {
            writer.WriteLine("220 localhost -- Fake proxy server");
            for (string line = reader.ReadLine(); line != 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");
                        client.Close();
                        return;
                    default:
                        writer.WriteLine("250 OK");
                        break;
                }
            }
        }
    }
}
 
I admit that the above code is simpler and probably less elegant than yours, but at least it helps understanding how the whole thing works and, the timeout setting avoids ending up with a bunch of sockets idling and doing nothing Smile | :)
 
HTH

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