Note: This might be related to the problem here: https://support.mozilla.org/en-US/questions/1268857, but I'm not sure. Short version: I sometimes see this error (see image) in the message body when trying to move a group of messages to a different folder. The presence of the error seems to disrupt moving the messages -- when I don't see the "moving xx messages..." status at the bottom, I know one or more messages has the error. At least some of the messages move (haven’t checked them all) but this leaves behind the empty copy (subject but no content), which causes inbox management issues. It happens only with two of several accounts, the ones normally accessed on other devices first. All accounts use POP with mail left on the server. Thunderbird version is 45.7.0. Details: I manage several email accounts in Thunderbird and like to keep a lot of email. My typical method is to move messages from the inbox to separate folders. (Keeping the inbox full causes various problems.) I use POP3, leaving messages on the server for several months at a time and selectively delete locally. I do that because I want a copy on different computers for different reasons and don’t want to synchronize them. The problem involves only two accounts, the two that are regularly checked by someone else, also using POP3. In only those two accounts, which are on unrelated servers (one Yahoo and one Runbox), when I try to move messages from the inbox to another folder, the email subject remains on the inbox list, but with an error message instead of content. It happens only with some messages. It seems to be a minority of them. For the messages I’ve investigated, it has copied the message to the intended folder, but not removed it from the inbox. There are too many to check all of them. This wouldn’t be a huge inconvenience, but I cannot highlight those messages and delete them as a group. I can delete them individually, but there are many hundreds of them. I can’t easily determine which were copied to another folder, but I hope they are there and I just delete them individually when I have time. Leaving them in the inbox prevents me from easily managing the old messages. I can’t, for example, highlight and take action on any group of messages containing those. All other email functions seem to work. The empty messages show up in searches by subject (which is visible even when the content isn’t). Since it’s only two of several accounts, the only two routinely accessed on other devices (two pcs and a tablet), I’m assuming that’s the key to the problem. I don’t know if it applies only to messages left on the original server or some other factors. I’d have to do some testing to figure out a pattern. It seems to be more with old messages, so they were probably read on the other devices and might or might not still be on the server. All devices are set to leave mail on the server. Other than it generally being older messages, I don’t see a pattern. The only other clue is that the messages were probably viewed on the other devices first since they are for him, not me. I just keep a copy on my computer as backup and read very few messages. Thanks!