I have never had this happen, nor have I heard of it happening to anyone else. I'm sure it HAS happened, but I don't think it's a common problem with Thunderbird.

My first thought is to blame an anti-virus program, which very well could have, rightly or wrongly, detected a virus in the mail fold file. It might have quarantined the file. Can you check your AV, if any, to see its recent action and if it has moved anything into quarantine?

A moral of this story, which I see you already know, is to backup your Profile regularly!

Thanks for the good ideas.

Co-incidentally, antivirus did quarantine a threat around that time. However, a scan of the restored file itself was virus free, and antivirus allowed me to copy and open it without intervention. So that is unlikely to be the explanation. In addition, Windows Defender logs tell me it was a trojan in files that another user downloaded. That user does not have permission to access my AppData files.

The other point is this was a copy of my sent mail folder. Anything in it I generated, I inserted from a file that was in my system and already scanned, or I replied to or forwarded. The first two categories could not have been infected. The third could, but only if an incoming message was infected, and in that case the inbox should have been quarantined.

Now that does bring up an odd coincidence. Yesterday, my inbox was corrupted. The index was pointing to the wrong messages. I fixed that, but did not think it could be related to what has happened. I don't know if a specific message triggered the issue, and I routinely delete unimportant messages, so it would be hard to recreate the situation.

So: it is barely possible that an incoming message somehow evaded detection both by gmail and my antivirus (Windows Defender). It then trashed my sent mail folder some time after I sent it on to unsuspecting others. Very far fetched. Do you have any other thoughts?

Thanks.