With the former, I'd expect a large number of those messages to be
blocked/discarded (it's common to do call back verification, or just
simply check that the sender domain exists).
Checking that the sender domain exists is quite common, cheap, and has
a very low failure rate.
SMTP callbacks are one of those bad ideas that just won't go away.
They're quite abusive (consider the 95% of mail that is spam with
someone else's return address) and don't work, since your idea of what
I should say in response to your MAIL FROM and RCPT TO commands is
probably not the same as my idea of what I actually do say.
R's,
John
PS: The wiki at http://wiki.asrg.sp.am has a taxonomy of anti-spam
techniques, both the good and the bad. See if your favorite or least
favorite is listed there, and if not, please add it. If you don't
have a login to edit, just drop me a note and I'll give you one.
_______________________________________________
Asrg mailing list
Asrg(_at_)irtf(_dot_)org
http://www.irtf.org/mailman/listinfo/asrg