Email issue on iMac

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

Any change?

Hmmm, going to be difficult as POP usually erases things on the Server & Rebuilding any account would erase any eMails not on the server.

Do you have a Time Machine backup to Restore to/from?

https://support.apple.com/guide/mac-help/revert-to-a-previous-macos-version-mh15216/mac

Any Luck?

Is it an IMAP account or POP?