Thunderbird 3 insists on moving message receipts (with the subject "Read:") into the Sent folder. This never happened with any other client I've used with this IMAP server. I can't find anything about this behavior in Thunderbird's settings?
To clarify, if I manually move them back to Inbox using another client, they stay. As soon as I open/resync Thunderbird, they all appear back in the Sent folder (they appear there to all clients).