Software recommendation: Thunderbird alternative, that can import TB's data

tl;dr Looking for a reliable alternative to Thunderbird that

• can import (or read) all emails from Thunderbird (only emails is fine)
• support local account (read localhost mail, from /var/mail/...)
• option: has clean configuration files (not like prefs.js)

Longer shot:

after upgrading from Ubuntu 18.04 to 20.04, the newer Thunderbird 91+ doesn't support reading local emails anymore.

Not only that, but the Inbox that was linked to that "local" email account is not showing anymore (data is still here).

I tried messing again with the (oh my...) pretty-mezzy prefs.js, but, this time... giving up.

TB's recommendation is to create a local POP (or IMAP) server, but, if possible, I'd rather do a migration to something reliable, clean, and certainly lighter, that reads local mail directly.

Time for a change.

Thanks.

Got this from the TB folks

https://forums.mozillazine.org/viewtopic.php?t=3086614

And for the time being, just did

Just to clarify to other people, you only need to install mailutils-pop3d and restart your system. In Thunderbird just add your mail as a normal mail account, user@localhost and your login password"

(and actually that was straightforward ; apt install and TB account config)

Still opened to recommendations, though.