User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.102 Safari/537.36

Steps to reproduce:

It would appear that some bright spark has decided to remove the ability to revalidate a certificate when the email provider's certificate expires.

Actual results:

No easy way to revalidate certificates when sending email

Expected results:

An oppotunity to revalidate certificate without having to navigate to each email address and re add the server. By the way there is a good chance that all your old emails are going to be not avalible when you do this. Apple has done the same thing and this appears to be an industry wide problem where people are getting fixated of securing certificates.

I believe you are looking for a way to add a certificate override for invalid TLS server certificates.
If that's correct, the issue is known and being worked on, right?

Correct, we're working on fixing it. For SMTP, bug 1665577.