There never appears to be a solution as they simply abandoned their topics. We must assume the answers they got solved their issue, but they never bothers to report it.

The same general advice applies to you, as you have not offered any information regarding the problem and your accounts that is unique.

If you get mail on your phone, this is often marked as read by either the mail client on the phone or the mail server (I think gmail does this for POP phone accounts) When you have an IMAP account the problem is that status is syncronised to the Thunderbird instance about the same time as the mail is downloaded.

SO poo0rly programs product like anti virus program force their code into Thunderbird with spam and scam buttons and filtering. These have long been the source of various random and vague issues. like message arriving marked read. Every mail not in the server address book being marked as spam and other things such as mail simply not downloading. Hence the standard suggestions of Thunderbird troubleshoot mode (this forces all addons out, even the hidden ones) and safe mode with networking in the operating system too see if it helps. For some reason accepting that the security software you have chosen is not a good choice going forward is just to hard for folks, so we let them work it out for themselves. by disabling it in the operating system safe mode startup.

I do think one of the two modes will shed light on your problem as well.

Note the most mentioned anti virus problem child of late has been bitdefender. But that is it is having it's turn.

This is the only machine with these email accounts Every account is pop3 with nothing else attached. There is no antivirus on this machine. The problem showed up in the past 2 vers, and it wasnt there before the upgrades