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

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

System.Net.Mail only support "explicit" SSL, as explained 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 that I don't have any control over, and it's failing. How can I know for sure that it's failing because the server wants an "implicit" SSL connection? What test can I do from the client side?

A. Rex
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Corey Trager
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1 Answers1

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Ordinarily this is governed by convention. SMTP running on port 25 (the normal case) uses explicit SSL. SMTPS running on port 465 uses implicit SSL. Mail submission running on port 587 uses explicit SSL.

To tell for sure, telnet to the port, as in "telnet mail.example.com 25". If you see a plain text banner where the server identifies itself, then you are dealing with explicit SSL. If you connect successfully and see nothing, then you are dealing with implicit SSL.

Andru Luvisi
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  • I have made tests to a mail server that listen in port 465. First and I have trying to send email using explicit ssl and was unsuccessful. When I thest manually using telnet the banner showed up in plain text, however the way to successful send email was using implicit SSL. I supose that what you say is not fullfiled in all cases. – Luis Carlos Jun 03 '20 at 07:29