I cannot receive email to the server with an SSL certificate applied. That is to say, emails are rejecting at the server, but only if the SSL certificate is on.
Back story is that I had to do a Time Machine restore on the server recently. Wow, TM is a really weak backup solution for a server! There are no shortage of problems, but receiving email is the top priority right now.
I've reissued the certificate(s) and replaced the existing ones. One applied in the Server.app > Mail, the intermediates imported to Keychain > System. I've rebooted, turned things off and on, verified everything I know to verify, but I can't see anything wrong.