One thing to keep in mind if you use Thunderbird is that their certificate verification library (Mozilla::PKIX) really, really hates self-signed certificates. S

One thing to keep in mind if you use Thunderbird is that their certificate verification library (Mozilla::PKIX) really, really hates self-signed certificates. So, if you're using a self-signed cert on your mailserver's submission port or to encrypt IMAP traffic, Thunderbird will simply fail to work. Unfortunately, I know of no way around this. I just gave up and started using Claws in the meantime.