I updated my iMac to Catalina today and most things seem OK.
I use Thunderbird as my email client (updated to latest) but this seems unresponsive.
I get notifications of new messages, but there seem to be no unread messages. There seems to be no way to force downloads – even Get Messages doesn't seem to work.
My Gmail account is set to use IMAP and checks for new messages every 30 minutes.
If I access Gmail in Google Chrome the messages are there.
Everything worked OK in Mavericks.
Further Part of the problem seems that Unread Messages are no longer shown in BOLD although this does not seem to be the whole problem as QuickFilter Unread did not initially show any messages.
Best Answer
Apparently this is a known issue.
I stated "Thunderbird … (updated to latest)" - BUT I was mistaken! The Update only updated to a version of 60. There seems to be no Update option.
There is apparently a version of 68 which does NOT have this problem BUT disables most of my Add-ons.
It seems I have a choice of displaying unread email OR working Add-ons (which is the whole point of Thunderbird).
Maybe I should try another email client (I find the side by side display of Mail unusable)