Thought this might be useful.
I have a Linux Reseller Account, with a number of mail servers, and have a large number of admin accounts for support. I use Mozilla Thunderbird for my own mail, and wanted to try out the security features.
I use TBird 2.0.0.4 and if I select "Use TLS, if available" in Account Settings > Server Settings > Security Settings I had a problem with mismatched domain names. The mail server is on one domain, but the server is signed by the domain name of the computer on which my reseller server resides - e.g the mail server is mail.mydomain.net, but the certificate is signed by computername.hostdomain.com. This would throw up a mismatched domain name error, so I would have to turn that security setting off. This doesn't happen if you have a
VPS Hosting or Dedicated server account.
The solution was to install a Thunderbird extension called Remember Mismatched Domains. When the error comes up you can choose whether to accept the certificate permanently, temporarily or not at all. After checking it is valid, I can select permanently and then it never asks again. It also works with SSL, and I can turn on "Use Secure Authentication".
Thunderbird is available here -
http://www.mozilla-europe.org/en/products/thunderbird/
And the extension here -
https://addons.mozilla.org/en-US/thunderbird/addon/2131