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Posted 20 Nov 2011
Licenced CPOL

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);

while (true)
    TcpClient client = listener.AcceptTcpClient();
    SMTPServer handler = new SMTPServer();
    Thread thread = new System.Threading.Thread(new ThreadStart(handler.Run));

The protocol has the following messages in this example:
S: 250 Ok

S: 250 OK

S: 250 OK

S: 354 Start mail input; end with <crlf>.<crlf>


S: 250 OK
C : Quit

Basically the Run method contains this:
public void Run()
    Write("220 localhost -- Fake proxy server");
    string strMessage = String.Empty;
    while (true)
            strMessage = Read();
        catch(Exception e)
            //a socket error has occured

        if (strMessage.Length > 0)
            if (strMessage.StartsWith("QUIT"))
                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);

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!


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


About the Author

Netherlands Netherlands
No Biography provided

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Comments and Discussions

Questionwhat about the mail queue? Pin
Pal Sayantan14-Mar-18 23:47
memberPal Sayantan14-Mar-18 23:47 
GeneralMy vote of 1 Pin
Jason McBurney5-Mar-18 6:02
memberJason McBurney5-Mar-18 6:02 
NewsSure would be handy to know... Pin
countrysideflair23-May-13 14:15
membercountrysideflair23-May-13 14:15 
GeneralMy vote of 2 Pin
perilbrain16-Feb-13 1:57
memberperilbrain16-Feb-13 1:57 
GeneralRe: My vote of 2 Pin
Jason McBurney5-Mar-18 6:01
memberJason McBurney5-Mar-18 6:01 
Suggestionthanks i am now going to design powered relay Pin
smtpserver.in7-Nov-12 20:47
membersmtpserver.in7-Nov-12 20:47 
Questionplease some one help me how to test dkim with smtpserver relay Pin
smtpserver.in22-Oct-12 4:12
membersmtpserver.in22-Oct-12 4:12 
Questionsmtp server in c Pin
Igede Hardisurya Budiana7-Oct-12 15:19
memberIgede Hardisurya Budiana7-Oct-12 15:19 
QuestionOne more update to my alternate version - full source Pin
Mark Harmon11-Sep-12 10:15
memberMark Harmon11-Sep-12 10:15 
AnswerRe: One more update to my alternate version - full source Pin
ObiWan_MCC17-Sep-12 3:42
memberObiWan_MCC17-Sep-12 3:42 
QuestionAnother alternate - full source Pin
Mark Harmon11-Sep-12 8:07
memberMark Harmon11-Sep-12 8:07 
GeneralRe: Another alternate - full source Pin
partha chakraborti14-Jan-15 19:46
memberpartha chakraborti14-Jan-15 19:46 
AnswerRe: Another alternate - full source Pin
Member 1185245119-Jul-16 6:05
memberMember 1185245119-Jul-16 6:05 
SuggestionSome suggestion to improve your tip Pin
ObiWan_MVP3-Sep-12 6:13
memberObiWan_MVP3-Sep-12 6:13 
GeneralReason for my vote of 5 Thanks man! It works but my firewall... Pin
finkos24-Nov-11 8:31
memberfinkos24-Nov-11 8:31 
GeneralYou did not define the server variable in your code. What is... Pin
finkos22-Nov-11 14:51
memberfinkos22-Nov-11 14:51 
You did not define the server variable in your code. What is it?
GeneralReason for my vote of 5 good - i'll give it a try! Pin
johannesnestler22-Nov-11 7:13
memberjohannesnestler22-Nov-11 7:13 

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