TorBirdy should have an option to distrust all certificate authorities

The certificate authorities (CA) mechanism for validating TLS has proven to be rotten to the core in several occasions. While browsing the web, not relying on it is quite hard as you might be presented a different certificate on each webpage that you are visiting. But in the case of email, you basically always use the same and only one: the certificate from your email provider. So trusting all those CAs by default and allowing so many possible man-in-the-middle attacks is not really needed for usability.

TorBirdy could have an option to distrust all CA authorities by default and only rely on custom certificates (exceptions).

Users could, in the worse case, do TOFU authentication (trust on first use) and be guided on how to do so. At best their provider can give them better ways of authenticating their certificates. Riseup is proposing this on their website for example: https://help.riseup.net/en/certificates.