Outlook - Mailbox Full Message after storage quota increased (or removed)

I recently experienced an issue where multiple users for a mailbox were getting a "mailbox full" message in Outlook. When the message first appeared, I removed the storage quota on the mailbox, but after a server restart (and of course restarting the outlook clients), they were still getting prompted with the same message.

The solution ended up being to remove the .ost file for this particular mailbox (default location is %localappdata\Microsoft\Outlook) and then restarting the Outlook client (you will need to close Outlook before it will let you delete the .ost file)

The .ost file will automatically be re-created when Outlook is opened, and mail will then be downloaded back into the .ost file from the server.