I don't think that's really the case, is it? At least, not in any formally-specified way. Modern email clients will extract metadata for things like airline reservations, shipping trackers, ICS calendar invites, etc, and give you live tiles specific to that time-sensitive info, but it's very clearly supplementary and at least in GMail none of it is pretending to be part of the message itself.
Doesn't GMail pre-fetch those images and then cache them for the duration of the email being in your inbox, so as to defeat tracker dots, read receipts, etc?