We've made some improvements to the mailbox screen. You can now wee which messages have been flagged, and which messages have high or low prirorities (as defined by the sender).
You can change the flag status of a message using the "Mark flagged" or "Mark unflagged" items in the Action menu.
There also some "flag searches" at the top of the messages list, which allow to quickly show only the unread messages, the read messages, or the flagged messages.
These are all currently on the beta server.