Smart Mailboxes stay up-to-date automatically as you send and receive mail. Use the default Smart Mailboxes provided by Mail (Unread, Flagged, Today, Last 7 Days) or create your own using single or multiple rules. With Smart Mailboxes, your email can be in many places at once, even if you cant.
A smart unread box? Mmmh, that sounds deliciously Opera M2-ish. Yay!