You’ve likely seen Schema markup in search results that have ratings, author information, etc. Well, Schema markup for email places click-able calls-to-action in the subject line of your email, e.g. view order, RSVP, complete purchase, etc. But there’s one caveat, it only works with Google’s Gmail. So before you start rushing to implement this, you should make sure you have enough clients with Gmail accounts to justify it.
Read the full article here:
An Introduction to Schema.org Markup for Emails