The updated Mail app is Version 7.0 and is labeled as build 1822. The OS X Mavericks 10.9.0 Mail app, for comparison, is Version 7.0 with build number 1816.
The update is said to fix issues with Gmail, Smart Mailboxes, and stability. Testers are asked to:
- Use Mail with your usual Mail accounts, including iCloud, Gmail, Exchange, etc.
- Send Mail messages
- Receive and check for new Mail messages at the intervals you expect it
- Read and move Mail messages to folders on your mail server, folders on your Mac, smart folders.
- Delete Mail messages as appropriate
- Undo move or delete actions with your Mail messages
- Mark messages read/unread in both your mail provider’s webpage and Mail and verify they stay in sync.
It's unclear if Apple will release a standalone patch to the current Mail app or include the update in OS X Mavericks 10.9.1.