Some of my servers that are only for internal/personal use have SSL certs with a mismatched hostname. The cert for my mail server is issued for the CNAME used for my mail server, not the actual hostname. Of course, this means that Thunderbird gives me some annoying errors because they’re worried:
Luckily, there’s an add-on called “Remember Mismatched Domains” that adds a simple “remember this decision” check box, much like the one now found in Firefox. Problem solved!