Thunderbird says the certificate for my mail address is not valid for that server, and thus email will not work.

- Who is your email provider? - What anti-virus/network security/VPN software do you use, if any? Certificate errors can be caused by several things, including: - Wrong server settings. You should confirm that they are correct. The settings are usually found on the provider's web site, or you may need to contact them - A-V software can interfere with secure connection. Norton and McAfee are especially notorious for this. You can disable them or create an exception for Thunderbird or turn off all email scanning features to test. If you use Windows you can restart in Safe Mode with Networking to test. - Your provider may truly be having server or certificate problems. Only they can fix this. You can contact them to report the problem (they may already be aware) and to find out the status.