Email on the Apple Watch is a curious thing—you’re actually discouraged from engaging too deeply with your mail (but that’s not a bad thing). The Apple Watch ties into the Mail app on the iPhone primarily to help you read and sort your Inbox, as well as dash off quick replies. You still need to write new messages on the iPhone.
Your Inbox(es) and a couple of messages are displayed. Even in a small space, the app delivers quite a bit of information: