I had to configure Thunderbird for a coworker last week, connecting to the dreamhost mail servers, and it does validate the server certificates.
You have to either add an exception (works, but not ideal), or find the exact hostname which will pass Thunderbird's certificate validation (dreamhost uses a wildcard certificate for * .mail.dreamhost.com, so after digging on its wiki you find out that the correct alternative is in the form of subn.mail.dreamhost.com, where n is the same number on the mail server's real hostname, which you cannot use because it has two components where the * can match only one).
(Edit: can't make the asterisks show up correctly.)
(Edit: misread MTA as MUA. Yeah, most MTA's can't validate the certificates.)
You have to either add an exception (works, but not ideal), or find the exact hostname which will pass Thunderbird's certificate validation (dreamhost uses a wildcard certificate for * .mail.dreamhost.com, so after digging on its wiki you find out that the correct alternative is in the form of subn.mail.dreamhost.com, where n is the same number on the mail server's real hostname, which you cannot use because it has two components where the * can match only one).
(Edit: can't make the asterisks show up correctly.)
(Edit: misread MTA as MUA. Yeah, most MTA's can't validate the certificates.)