From the Apple Watch User Guide: ?Delete email. If you?re reading the message in Mail on Apple*Watch, firmly press the display, then tap Trash. If you?re looking at the message list, swipe left on the message, then tap Trash. You can also delete a message from its notification?scroll to the bottom of the message, then tap Trash.
Deleting a message on Apple*Watch also deletes it from iPhone.
Note: If your account is set to archive messages, you?ll see an Archive button instead of a Trash button.?
Excerpt From: Apple Inc. ?Apple Watch User Guide.? Apple Inc., 2015. iBooks.
This material may be protected by copyright.
Check out this book on the iBooks Store:
https://itunes.apple.com/us/book/apple-watch-user-guide/id985786488?mt=11&at=10l3Vy