I can't work out a definitive answer on this, but from searching I find two links which seem to indicate to me that a server (in this case it's MS Exchange as per the links) can have different certificates in place for https than for secure smtp/TLS.
http://technet.microsoft.com/en-GB/library/bb851505(v=exchg.80).aspx https://www.sslshopper.com/article-how-to-use-ssl-certificates-with-exchange-2007.html
I have an issue which no-one has been able to help with here and this question is a follow on, in that I am coming to the suspicion that my first problem is that my machine trusts the https certificate, but not the one being used for smtp/TLS. But what I'm asking now, is that even possible?
Going through the diagnostic steps here shows me that the certificates in use when I access my mail server's web interface through https are fully trusted. However when I look at the debug of my c# process it is stating a completely different certificate issued by one of our servers to it's self (the server on which exchange is installed).
So... any one know if it's possible that I am thinking along the right lines... is it possible that when I do an https connection I get one certificate and when I use the .net SMTP client I get a completely different certificate (from exactly the same address, but I assume a different port)?