(A year since the mailbox/addresses panel reorg, and still I can't find where on the panel to do simple things like this. Am I thick?)
Though perhaps you are looking for removing a mailbox that isn't listed on the addresses panel at all? Or do you mean having a FTP/shell user but no mailbox for that user?
Perl / MySQL / HTML+CSS
In your defence, I guess it is easy to misread "delete a mailbox" for "delete an address plus mailbox"
If it was as easy as you suggest, I would not have asked.
Try "Creating a new email address" and see if the mailbox appears in the dropdown for the "deliver to this mailbox" section. If so, assign a bogus address to this mailbox and then after confirming that operation has completed, the mailbox should appear in the addresses list where you can delete it.
Otherwise contact support of course, since there doesn't appear to be any other way to resolve stray mailboxes.
It sometimes does, sometimes doesn't.
I fear you are correct.