Back on topic ... For the record; Mint 20's Evolution has completely screwed up my previous email settings (supposedly restored from backup) besides failing to deliver some messages that still come in on my other (Mint 17) laptop. Mint 20 is getting dumped. It seems backward compatibility is a backward concept.
I have a very complicated configuration, including using PGP to sign and encrypt certain emails, and I merely backup/restore the ~/.thunderbird directory.
Now, sometime around version 78 Mozilla introduced some breaking changes. They then skipped a bunch of versions, for reasons unknown to me. But, if your older Mint was using an older version of Thunderbird, it may have broken stuff.
It took me about an hour to get things squared away with the new version, needing to mostly restart from zero. I did have backups for my key files, which saved me some time and provided continuity.
If that's the case, it's probably not Mint's fault. It was gonna happen eventually.
Thanks for the prayer, Condobloke. My wife's health problems are somewhat relieved but not solved, during further investigation; which involves many visits to hospitals & clinics; mostly waiting around for tests, results, or just to see someone. The whole world is understaffed & overworked. I'll get back to whining about computing soon.