Is there a way I can tell if the SMTP server expects to connect to the client using "implicit" SSL versus "explicit" SSL?

Question

SSL can be "explicit" or "implicit", as explained by this link:

http://help.globalscape.com/help/secureserver2/Explicit_versus_implicit_SS.htm

System.Net.Mail only supports “explicit” SSL, as described here:

http://blogs.msdn.com/webdav_101/archive/2008/06/02/system-net-mail-with-ssl-to-authenticate-against-port-465.aspx

So, I'm trying to use System.Net.Mail to connect to an SMTP server on which I have no control and it does not work. How can I know for sure that it is not working because the server wants an “implicit” SSL connection? What test can I do on the client side?

Answer 1

This is usually regulated by convention. SMTP running on port 25 (normal case) uses explicit SSL. SMTPS running on port 465 uses implicit SSL. Sending mail running on port 587 uses explicit SSL.

To say exactly, telnet to the port, as in "telnet mail.example.com 25". If you see a text banner where the server identifies itself, then you are dealing with explicit SSL. If you connect successfully and cannot see anything, you are dealing with implicit SSL.