I just upgraded from Lion to Mountain Lion. I have eight email accounts set up in Mail.app. Some of my SMTP servers support SSL and some don't. But now Mail.app turned SSL on for every server (except the one using a custom port) and it won't let me turn it off. When I uncheck "Use Secure Sockets Layer" in the Advanced settings, it just automatically rechecks itself. How can I get it to go off and stay off?
I figured out a workaround. I took the clue from my account that uses a custom port. For the accounts in which SSL doesn't work, I checked "Use custom port" and tried each of the standard port numbers until I found one that worked. Which the custom port selected I can successfully disable SSL.