and isn't
considered to be that much better than C/R (it doesn't clutter a
forged-sender's mail box, but it can bog down a forged-sender's mail
server with verification requests).
Well, it may be. I know, however, that a lot of people is doing this at the
MTA level in order to reject mails with forget sender.
Also, SAV's drawbacks may probably be mitigated by caching the results.
Except that many ISP's will blacklist you for probing them in this
manner. It even mentions it right in the postfix documentation.
http://www.postfix.org/ADDRESS_VERIFICATION_README.html#limitations
