This is new to me, I've been off and on trying SSL email for years with Dreamhost but I always give up when it comes to trying to download and edit certs. No one seems to just link to the cert so I can import it, and it wouldn't work too well anyway. setting your server as your specific server isn't the best plan because they could change URL one day. Using mail.yourdomain.com is the only acceptable answer, in thunderbird this causes a domain mismatch error.
This addon lets you accept the *.mail.dreamhost.com certificate for your domain permanently!
Just trying to help. If you see similar topics on the net about thunderbird and SSL mismatch, link to that addon. Thunderbird 3 will fix this issue.