I don't think I can help but what is the file size of the Trash Folder that won't compact? (triple dots beside 'new message' and 'Show Folder Size' which is a toggle). Are you using the routine compaction setting or trying to do it manually?

Compacting is seemingly done automatically (weird because I usually must do manually). I checked the "size" c/o Properties of the trash folder but the size given is the size of the content (like 0 if empty or 450 k for small trash), not the "folder size" as such.

So, would you give me the size of the Trash Folder in question, please?

As said, you can't see the "size". only the size of the content. So, is 0 or 500 kb or anything, depending on what is in the trash. Note that the warning comes without asking to compact, while trash is working OK and ... can be compacted manually. So a warning it seems without any reason. Earlier it was only in the IMAP email, now starts in one of the POP emails.

I don't understand your answer. When you choose to see the Folder sizes, what is the size beside the trash folder that you are having trouble with? And how many items are in there? See attachment

I already explained. The size shown is the size of the items in the trash folders. As the trash actually works despite the "alarm", the trash can be empty (0 kb) or with like 3 small items 500 kb. See as example, in one of the affected emails: 17 items in trash 769 Kb. After emptying trash and compacting successfully (yes!) folder 0 items 0 kb. At the same time the (false) alarm comes... while everything is fine... Must be some software bug

you should not be compacting the trash really, just empty it periodically using the menu entry and the file holding trash will be deleted and a new one created.

The trash can be compacted either directly or from the header of the email (then all folders are compacted). The trash folder cannot be deleted. In my case deleted mails go to the trash, the trash can be emptied and compacted. Despite the "warning"

you should not be compacting the trash really, just empty it periodically using the menu entry and the file holding trash will be deleted and a new one created.

The Trash Folder is cumulative in its size, even though it can be emptied - and compaction does return the value to zero when no emails are present. I haven't yet reached the file size for automatic compaction to take place but manual compaction does remove the 'blank space'. Removing the emails does not zero the content size.

The only way round the questioner's problem, I see, is to delete the data file in the profile belonging to the relevant trash folder (which I did in the early days before I understood compaction) with success.

I can't, however, give the questioner a set of instructions as I need the trash file size so I can make sure he deletes the correct file, based on size, as it may be that he doesn't have his file extensions visible.