As they have done with other apps, HTC has moved the Sense Mail app to the Play Store. This allows for easier future updates to occur, since a firmware update takes longer to be pushed to users instead of an app update.
There are no new features for this update, except that all future updates will now come from the Play Store. The Mail app works with Gmail, Yahoo Mail, Outlook, Exchange ActiveSync and other IMAP or POP3 email accounts, but only runs on newer Sense-based devices. So keep that in mind.