Deleted messages are simply marked as deleted and not permanently removed until the folder is compacted. Right-click the Spam and Trash folders, Compact.

Hi sfhowes,

I tried that, but didn't help. Both mail entries sometimes first disappear and then come back when reentering the folder, and other times they remain, when choosing Compact. Is there another chance to get rid of them?

But I detected one small progress: On one of both PCs (the one at my job, is Linux) both mail entries have disappered without coming back. The only thing I did there, was closing and reopening Thunderbird, without trying to delete them another time (and without choosing Compact). I wish, this were so on my private PC (is Windows 10) too. But it isn't.

BR tkansgar

In the event that the folder is corrupted and not removing deleted messages upon compacting, you could open the profile folder (Help/Troubleshooting, click Open Folder), close TB, and delete the mbox and msf files for Trash and Spam, i.e. Trash and Spam with no extension and Trash.msf and Spam.msf, then restart TB, which will automatically create new, empty Trash and Spam folders.

Hi sfhowes,

I tried this too, but again it didn't help. But something has changed a little bit. The mail entry in the spam folder behaves as before (described above). But the one in the trash folder don't disappear and come back as before, but remains permanently without any changing. Is there another thing, I can do?

Not all of the files, you mentioned, did exist. There were no files Trash and Spam without extension. Only .msf files existed: Trash.msf, Spam.msf and Spam-1.msf. Additionally Junk.msf and a folder Spam.sbd, I both deleted too (they all now are in the trash of my disk).

BR tkansgar

If the message appears in the Trash folder after deleting the mbox and msf files, then that must be a result of the folder re-syncing with the IMAP folder. So, you would have to empty Trash from webmail, since TB only shows what is on the mail server.

Hi again,

in the webmail front end both folders are empty (I wrote it already in my first posting here). If there is some garbage, I can't see it. Don't know, how to cleanup the folders. The menu entry to empty them is disabled.

But then: I started Thunderbird in a Windows VM on the job PC, and there both mail entries also appeared. So you may be right. But in that spam folder two additional very old mails falsly detected as spam by that thunderbird appeared. I deleted just these two and that strange mail entry too, and then also from the trash folder. In this Tunderbird instance none of them came back. Both folders remain empty. On the job PC itself this was so before (I wrote it already) and is now too. Then I closed all three Tunderbird instances and opened each one alone, first in the VM, then on the job PC and last on my private PC. In all instances both folders are and remain empty now. I don't know, how this could happen. In the Web front end I did nothing, in the VM I only did, what I did before on the other PCs. I will watch these folders next days.

Many thanks, sfhowes, for your help!

BR tkansgar