I have a user who has a former employee's mailbox listed within their Outlook. The former employee's mailbox is listed as "unlicensed" in Microsoft 365 admin center. When the user clicks on the mailbox to try to access it, they receive the error "Cannot expand the folder. The set of folders cannot be opened." We want to remove this mailbox from the user's Outlook. Unfortunately the user isn't in the office very often, so I have forgotten exactly which troubleshooting steps we've already tried. Additionally, I don't have a lot of recent experience with Outlook troubleshooting and none at all with Exchange administration in Outlook 365. I remember though that it wasn't an obvious fix (hence this post). Any assistance would be appreciated.