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Create a simple SMTP server in C#

By , 20 Nov 2011
 
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!

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

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 roosrj
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GeneralMy vote of 2memberperilbrain16 Feb '13 - 1:57 
Suggestionthanks i am now going to design smtpserver.in powered relaymembersmtpserver.in7 Nov '12 - 20:47 
Questionplease some one help me how to test dkim with smtpserver relaymembersmtpserver.in22 Oct '12 - 4:12 
Questionsmtp server in cmemberIgede Hardisurya Budiana7 Oct '12 - 15:19 
QuestionOne more update to my alternate version - full sourcememberMark Harmon11 Sep '12 - 10:15 
AnswerRe: One more update to my alternate version - full sourcememberObiWan_MCC17 Sep '12 - 3:42 
QuestionAnother alternate - full sourcememberMark Harmon11 Sep '12 - 8:07 
SuggestionSome suggestion to improve your tipmemberObiWan_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
Sign In·View Thread·Permalink
GeneralReason for my vote of 5 Thanks man! It works but my firewall...memberfinkos24 Nov '11 - 8:31 
GeneralYou did not define the server variable in your code. What is...memberfinkos22 Nov '11 - 14:51 
GeneralReason for my vote of 5 good - i'll give it a try!memberjohannesnestler22 Nov '11 - 7:13 
Last Visit: 31 Dec '99 - 18:00     Last Update: 21 May '13 - 6:39Refresh1

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About Article
A simple basic SMTP server for testing purposes
Type Tip/Trick
Licence CPOL
First Posted 20 Nov 2011
Views 29,788
Bookmarked 13 times
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