Delete ghost folders in Outlook

Here’s a pretty annoying issue that took me a while to fix! My client had folders in his mailbox (standard IMAP) that he couldn’t delete. No matter how many times he did a right click > delete or dragged the folder elsewhere in the mailbox tree, the folder kept reappearing.

I took over but couldn’t get any conclusive results with either Outlook or OWA, so I spun up a VM, installed Thunderbird and configured the mailbox. Here, a change compared to Outlook: the problematic folders appeared in italics and in a lighter font color.

To see what the differences were between a normal folder and an italic folder, I used the Mutt CLI. After configuring (again) the mailbox on the tool and connecting to it, I opened the folder explorer by pressing the c key (change-folder) then ? to list all directories available on the server.

In the Mutt interface, I tested a few options and then found the magic key: TAB (or the key configured for toggle-subscribed).

This command allows you to switch between displaying “Subscribed” folders (those the IMAP server thinks you’re subscribed to) and the list of all folders actually physically present on the server.

That’s where the diagnosis came in: In “Subscribed” mode, the problematic folders were indeed present in the list. But as soon as I switched to the complete list of server folders (the result of the IMAP LIST command), these folders disappeared.

These folders no longer existed physically on the server, but they had remained registered in the subscription database (LSUB) of the IMAP account. Outlook and OWA, seeing a subscription instruction, tried to display or recreate them, but since the source had disappeared, they entered (I think) an infinite loop of “deletion/reappearance”. Thunderbird, on the other hand, identified the anomaly and displayed them in italics to signal that the link to the physical folder was broken.

To solve this via Thunderbird, it’s very simple: right click on the account, Subscribe, uncheck the ghost folders (Unsubscribe), and validate. If everything goes well, the folder in question will finally be deleted from the display.