You can't send email from your Apple Watch, but you can read any email sent to you.
While you can use your Apple Watch to dictate text message replies, the inability to edit text makes the same system ill-suited for longer and potentially more complex emails. So, there's no Siri for Mail, but there is a read-only version of the Mail app on your Apple Watch. You can see what's been sent your way, and can even mark mail as unread, flag it, and delete it. To respond, however, you'll have to hand off to your iPhone, iPad, or Mac. Here's how it all works!