[MlMt] How do I delete old email accounts?

[Muster Hans]

A domain I am no longer interested expires today and I am trying to 
delete the associated accounts from MailMate, without success.

I have emptied the appropriate mailboxes but when I right click on the 
desired accounts in the sidebar under Inbox, the 'Delete Mailbox' option 
is greyed out.

These accounts are already deleted via the ISP's control panel.

Currently baffled.