Currently, you can only reply to emails from the Apple Watch itself. You can do this by scrolling to the bottom of an email and clicking 'Reply' then using the above method as mentioned.
The Apple Watch does not currently support writing new emails. You are only able to do this through the paired iPhone. This is with the stock 'Mail' app. If you use a third party application such as GMail, you may be able to start a new email, but you would have to do this manually rather than through the 'Hey Siri' method. I say may as I haven't tested this myself as I only use the Mail app.
Hope this answered your question.