Have we already forgotten the craze for this kind of thing a few years
ago? Here's an illustrative blog post I pulled up with a quick search
on "smtp callback":
AOL was in fact doing "call back to port 25 on the sending IP" for a
while, I still have a rack of mail servers that ought to be outbound
only but which are EHLOing (and then rejecting everything) on port 25
because of this foolishness.
ietf-smtp mailing list