Do you have Thunderbird set to show deleted mails in the IMAP settings?

Right click the account, and selet settings Select server settings in the account and under When I delete a message, set it to Move it to and set a deleted folder.

Those are the settings that I currently have.

Let me describe further. I select an item in my Sent folder. I click on it and click Delete. The item disappears and then reappears and the Delete button has changed to Undelete.

What you are describing is what is supposed to happen with the option set to mark it as deleted.

Previously when I selected an item in my SENT folder and clicked DELETE, the item moved to the Trash folder. Now it stays in the SENT folder only the DELETE button has changed to UNDELETE.

Are you saying this is the way it should work?