How to validate SMTP server

Question

I am working on a project where I have to validate the given SMTP server i.e in a textbox user will provide the detail and then he will click on a test button. I want to check whether the server entered by the user is an Smtp Server or not?

Any Idea??

Answer 1

You might want to improve on this quick code with proper exception handling and maybe also setting the timeouts - it takes about 15 seconds to fail if it can't connect but that might be a limitation of the TCP/IP handshaking.

And sending a QUIT command as Curt suggested would be nice.

private bool ValidSMTP(string hostName)
{
    bool valid = false;
    try
    {
        TcpClient smtpTest = new TcpClient();
        smtpTest.Connect(hostName, 25);
        if (smtpTest.Connected)
        {
            NetworkStream ns = smtpTest.GetStream();
            StreamReader sr = new StreamReader(ns);
            if (sr.ReadLine().Contains("220"))
            {
                valid = true;
            }
            smtpTest.Close();
        }
    }
    catch
    {

    }
    return valid;
}

Answer 2

Attempt to connect to the SMTP port, and ensure you get a line back from it that starts with "220 " and contains the letters "SMTP". A typical example response would be:

220 prod.monadic.cynic.net ESMTP Postfix (2.5.5)

Then be polite and send "QUIT\r\n" to hang up.

You can do some further testing, if you like, such as testing that the user can likely deliver messages. For this, you'd send a HELO command with your hostname (or any string, really), a MAIL FROM command using the user's e-mail address, and a RCPT TO:<[e-mail]>. Most servers at that point will tell you if relaying is not allowed. (I'm assuming you're doing this from the computer from which you will later be sending mail.) So long as you QUIT after that, rather than issuing a DATA command and the message data, nothing will be sent.

Here's an example session, done from the shell using the "netcat" command, showing that my server exists, but will not relay mail for people from random IP addresses.

    $ nc prod.monadic.cynic.net. 25
    220 prod.monadic.cynic.net ESMTP Postfix (2.5.5)
    HELO cynic.net
    250 prod.monadic.cynic.net
    MAIL FROM:<[e-mail]>
    250 2.1.0 Ok
    RCPT TO:<[e-mail]>
    554 5.7.1 <[e-mail]>: Relay access denied
    QUIT
    221 2.0.0 Bye
    $