I think I did these steps: 1) Selected a message in Inbox (in the top account, it's Pop3, not IMAP, all other accounts are also Pop3) 2) opened that message (it was on other tab) 3) Messages/Create Filter from Message ... 4) Choose Folder ... (for auto moving messages from that sender there) 5) Run Now The result: 1) All messages from Sent folder are gone and the Filter wasn't created. 2) All new mails do appear in Sent, but the old ones are still gone. 3) I'm not sure how that happened, but I'm sure that I would NEVER delete messages in Sent folder manually (since it's the main reason why I'm using Thunderbird - I want to keep all Sent messages from all accounts in one place). 4) Sent items from other accounts did not disappear (it would be a disaster). Maybe it has nothing to do with the Filter, maybe somehow those messages were automatically deleted by some maintenance? But yesterday all messages in Sent were still there. Maybe it was deleted by Thunderbird update? (don't remember when, maybe the update was yesterday)