mail.app SSL errors when sending mail

Mail was unable to connect to server “XYZ” using SSL on the default ports. Verify that this server supports SSL and that your account settings are correct.

This server was working a couple of days ago, and after a security update, no longer does. There's no indication in any OSX logs about WHY the SSL on this server is suddenly unacceptable. IMAP still works fine to the same server.