When trying to delete mails over IMAP in Thunderbird, I got the following errormessage:
The mails to-be-deleted re-appeared and couldn't be trashedMailbox doesn't exist: Trash
[SOLUTION]
Create a folder called "Trash", next to e.g. "Inbox".
Note: In Thunderbird, the Trash-folder icon is slightly see-through if the folder does not exist, and is fully-opaque once you created the Trash folder.