I've had that issue, but I havn't followed up on it.
It's only happened to me, and I haven't had any reports for users.
From what I can tell, it's a bogus number, and you really don't have 5 unread messages. ZCS still puts new mail in the inbox, but when you read it, it doesn't go away.
I fixed it by right clicking on "Inbox" and choosing "Mark all as read". |