GMail recognizes certain metadata fields in the HTML source of an email and adds extra features to its interface in response. Most fields support ecommerce use cases (e.g. flight ticket, room reservation), but there are some that might make sense for emails sent by Wikimedia:
- Go-To Actions, basically link buttons that are visible even in the closed state of the email - could be useful for talk page messages and similar Echo notifications
- One Click Actions, basically buttons that are visible even in the closed state of the email and can be clicked one time to fire off an API request. Could be used for email address confirmation and probably many other things.
There is a registration process involved.