• Start witth imap folder using maildir as offline store containing some messages, folder A.
• Check the "cur" directory for folder A and make sure there are files present, typically one per message in folder A.
• Make sure "Delete to Trash" deletion model/scheme is configured for imap account.
• Make sure Trash folder of imap account does not use offline store.

1. Select folder A
2. Delete message M in folder A
3. Select Trash and open the just deleted message M.
4. Drag or move message M from Trash back to folder A
5. Select folder A and open message M
6. Repeat steps 2 to 5 several times
7. Note that for each 2-to-5 cycle, a new file appears in "cur" with an increasing number suffix. See the attachment (*) for an example after doing about 6 cycles..
8. Note also that the latest file to appear is the same as the others except it contains additional headers at the top such as:

From - Thu Apr 13 17:46:37 2023
X-Mozilla-Status: 0001
X-Mozilla-Status2: 00000000

This may be related to the fact that cached messages contain these extra 3 headers that are specific to mbox format and are not needed, per BenC, for maildir.

Edit: Note also that to get rid of these extra files you have to either manually delete all but the one with the highest suffix or do a repair on the folder. Doing a "compact" (only possible when enabled as suggested in Bug 1827973 ) doesn't remove the built-up/redundant files.

*) In the attachment folder A is actually called "Skrapppost" which means, I was told by Magnus long ago, "Trash folder" in Swedish. Just to be clear, folder "Skrapppost" is not my trash folder but just a normal folder.