If you add a new IMAP account to mail.app you often get all the hidden mailboxes listed to. Most webhosts uses control panel software such as cpanel, plesk, ensim, and netmin. All of these add "hidden" imap mailboxes. Which Mail.app shows. These boxes often have data show up so mail.app thinks I have new messages :-P
My current IMAP:
Inbox
Sent
.cppop.cache
.cpop.cache.msg
.mailboxlist
sent-mail.
How can I tell mail.app to hide mailboxes that are clearly marked "HIDDEN" with "." in front..? Options for this in Outlook, Entourage, Thunderbird, Eudora, but I can't find one in mail.app.