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Create a simple SMTP server in C#

, 20 Nov 2011
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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)

About the Author

roosrj

Netherlands Netherlands
No Biography provided

Comments and Discussions

 
NewsSure would be handy to know... Pinmembercountrysideflair23-May-13 14:15 
GeneralMy vote of 2 Pinmemberperilbrain16-Feb-13 1:57 
Suggestionthanks i am now going to design smtpserver.in powered relay Pinmembersmtpserver.in7-Nov-12 20:47 
Questionplease some one help me how to test dkim with smtpserver relay Pinmembersmtpserver.in22-Oct-12 4:12 
Questionsmtp server in c PinmemberIgede Hardisurya Budiana7-Oct-12 15:19 
QuestionOne more update to my alternate version - full source PinmemberMark Harmon11-Sep-12 10:15 
AnswerRe: One more update to my alternate version - full source PinmemberObiWan_MCC17-Sep-12 3:42 
QuestionAnother alternate - full source PinmemberMark Harmon11-Sep-12 8:07 
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.
SuggestionSome suggestion to improve your tip PinmemberObiWan_MVP3-Sep-12 6:13 
GeneralReason for my vote of 5 Thanks man! It works but my firewall... Pinmemberfinkos24-Nov-11 8:31 
GeneralYou did not define the server variable in your code. What is... Pinmemberfinkos22-Nov-11 14:51 
GeneralReason for my vote of 5 good - i'll give it a try! Pinmemberjohannesnestler22-Nov-11 7:13 

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