Does a certificate have to be valid to mail using CDOSYS and SMTPS?

Question

Due to a limitation on our SMTP provder's side, we're having to use System.Web.Mail (deprecated), which is a wrapper around CDOSSYS.

Because we'd like to avoid having to change multiple configurations if we switch providers at a later date, we set up an internal alias for our providers FQDN.

So, mailrelay.ourdomain.com -> mailrelay.provider.com.

When I try to connect to either our alias or the provider's IP, a COM error bubbles up: "The transport failed to connect to the server." If I connect to the provider's true FQDN, everything works as expected.

I've looked in Wireshark, and I can see the certificate being requested, but not much happens after that.

I'm wondering if anyone knows if CDOSSYS checks to make sure the requested host name matches the FQDN on the certificate and fails if it doesn't match.

I've tried searching for an answer to this question, but I can't seem to find it.

Answer 1

I can't find a definitive answer, but from what I can tell, yes, CDOSYS does require a certification to match an SMTP server's FQDN when using SSL.