[MlMt] Removed IMAP account in Sources, still appears in Mailboxes

[John Cooper]

I removed an unused IMAP account by right-clicking it in the Sources list and then clicking "Remove Source." But this account still appears in the Mailboxes list as a subfolder in each of the standard IMAP categories: Inbox, Sent, Drafts, Archive, Trash. This persists after quitting and restarting MailMate. How can I remove all traces of this unused account?