Noticed today this great trinket from Google: in their notification emails upon new G+ followers, the recipient can add someone to a circle directly from a dropdown in the email. (See below).
The HTML for the page clearly shows the related div
has been attached to the end of the body, suggesting that the div di not 'ship with' the email and then was displayed via in-email javascript.
Is this kind of interactivity available to any engineer / product developer? Is this only something that Google gets to do because they control Gmail services?