That is why I see no need to "setup the email account(s) in Eudora correctly and make sure it recognizes the avast certificate" as you suggest since there was never a need to do this in the past.
There was also no need to peddle on a bicycle in the past. Simply because they had no peddles. Things change. ;)

https://support.tigertech.net/eudora-cert

https://www.avast.com/no-no/faq.php?article=AVKB91