I just created a new email account in Thunderbird. I've used T-bird for 15 years. Normally, when I highlight an inbox email and then press the keyboard "Delete" key, the message is sent to Trash. But on this new account, when I press Delete key, the message briefly disappears, but then comes back into my Inbox. It doesn't go to Trash.

I checked the Server setup and confirmed it is setup to move Deleted email to "Trash on xxx@xxxx.com"