I suggest you contact your email provider regarding the server certificate. All Thunderbird is doing is reporting the problem, but had no role in creating it.

Thank you David. You're right. After deinstalling and reinstalling I found out the server was down. :((

I have another question though.

I wanted to completely uninstall. Thunderbird and then reinstall so I saved the Profiles folder to a memory stick and used Revo Uninstall to uninstall the program. (Windows had made it a default program and so I couldn't uninstall it on Programs and Features.)

Revo Uninstall supposedly wiped the computer clean of Thunderbird but when I went to reinstall it the Profiles folder magically appeared. How so?

Thanks for your help

Technically, that's a Revo Uninstall question. I have used the product and it does a good job, but doesn't see everything. It cleans the registry, for example, but I can always find 50-100 missed entries. My guess is that the profile is left there because there is no executable code in the profile. :)