Automatically updated to 102.1.2 Same issue, BUT, when I started in troubleshooting mode(which I had tried before) and clicked on the Inbox for the missing accounts they slowly rebuilt themselves. Seems to be working OK now.

??? not sure what the problem was!

I suspect it is related to the %40 where the "@" sign would have been. When I first updated, one of my two IMAP mailboxes had vanished (Inbox and sub-folders) but a restart seemed to fix that (Phew!). Also, some of my filters were broken - I think the ones that moved emails to a sub-folder of that same mailbox. I had to reselect the sub-folders in the Message Filter dialog and re-enable the filter.

And in each case I think a username was embedded in the filter file and it had a "%40" in place of a "@". Perhaps because the same line has another "@" sign later on.

In my case it turned out I did not need to include the domain name in the username. So the fix was to edit C:\Users\jdoe\AppData\Roaming\Thunderbird\Profiles\6ab4si02.default\ImapMail\mail.mailservice.co.uk\msgFilterRules.dat

to change

actionValue="imap://jdoe%40mailservice.co.uk@mail.mailservice.co.uk/jdoe"

to

actionValue="imap://jdoe@mailservice.co.uk@mail.mailservice.co.uk/jdoe"

I am guessing the same kind of issue caused the entire mailbox to vanish after the initial install but that Thunderbird had some kind of self-healing mechanism to handle that.

Thank you for your reply. I just checked, and the %40 is still in the now working location.

just for some clarity. the appearance of the % number in place of letters in a URL string is normal and to be expected. they represent characters that on the web have special menings and they have to be "escaped" to prevent misintepretation.

A full list can be found here. https://docs.microfocus.com/OMi/10.62/Content/OMi/ExtGuide/ExtApps/URL_encoding.htm

By far the most common cause of what is discussed here of the contents of an IMAP account disappearing after an update is anti virus programs and software firewalls that block the new version from accessing the internet. So existing email is synchronized into oblivion as are folders other than the inbox and trash.

Generally the software in question gets either booted off the machine and everything works, or the user forces the Thunderbird exception to access the internet or after some days the product provider sends an update allowing the new version to work correctly. It used to be almost exclusively Norton's software firewall, but they are all as bad as one another these days.

Yes - I don't think it is the %40 per se. I submitted a bug report - and followed it up with a theory that I came up with after thinking about it some more. I suspect it is caused by an error in the Username that the user is using. Many mail servers expect your full email address (including @domain) but the one that was giving trouble seems to be working with only the part before the "@" (aka %40).

I think my account settings for the IMAP mailbox in question were wrongly configured with the full username@domain, and for some reason... that never mattered before the latest 102 release. On first launch after the update, Thunderbird failed to login to that IMAP server so the mailbox appeared to vanish. Some self-healing logic in Thunderbird (try without domain in username if it fails?) then caused it to be fixed in the account settings and so on re-launch, the mailbox re-appeared. However, the mail filters that were trying to move messages to sub-folders on that server were still broken so Thunderbird disabled them.

Not sure if this is correct, but if so it would explain why only a small percentage of users are going to get bit by it.

[Incidentally, the only AV I have installed in Microsoft Security Essentials, no third party firewall, and if the program had been blocked from the Internet my other two IMAP servers would not have been accessed without problems]

Do you agree with the assessment at https://bugzilla.mozilla.org/show_bug.cgi?id=1784598#c2 ?