Seconded, though it may have been a decade since I tried. I recall the GUI locking up and probably didn't complete the operation. Even ~2 years ago, I had simil

Seconded, though it may have been a decade since I tried. I recall the GUI locking up and probably didn't complete the operation. Even ~2 years ago, I had similar issues just trying to move a few hundred messages between folders on the same IMAP server. IIRC it worked, but takes a lot of patience. That time, I also didn't really care if it ate those messages instead of moving them. I wouldn't trust Thunderbird for more than a few dozen messages I could hand-verify. For anything more, use a dedicated tool that provides logs and can gracefully restart.