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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)

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About the Author

roosrj

Netherlands Netherlands
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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 
does anyone have a code smtp server in c?
 
I am very needed it.
 
please help me...
thank you
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 
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 

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