Hector Santos:
See RFC 2821 Section 3.3:
| 3.3 Mail Transactions
|
| .....
|
| ............. Despite the apparent
| scope of this requirement, there are circumstances in which the
| acceptability of the reverse-path may not be determined until one or
| more forward-paths (in RCPT commands) can be examined. In those
| cases, the server MAY reasonably accept the reverse-path (with a 250
| reply) and then report problems after the forward-paths are received
| and examined. Normally, failures produce 550 or 553 replies.
This is probably the most powerful concepts in RFC 2821 for anti-spam
operations. It opens the door for Mail From Validation and it offers
performance enhancement and overhead reductionn. We are probably among the
first, if the the only product in the market to have implemented this
recommendation.
The Postfix MTA does this by default since 19990510 (May 10, 1999),
according to the day-to-day log.
Wietse