Bart Schaefer <schaefer(_at_)brasslantern(_dot_)com>:
Suppose the (not uncommon case) of a sending MTA that is not the same host
as the MX for the sender's domain. Call these MTA1 and MX1. A message is
transmitted via MTA1 to the recipient's MX, which I'll call MX2. If the
sender mailbox is valid at MTA1 -- that is, MTA1 can successfully deliver
a DSN to that mailbox in the event of a 5xx response from MX2 -- where is
the requirement that the mailbox also be valid when presented as a RCPT TO:
at MX1?
RFC2821 says the Return-Path has to be the same address that was
presented to the MAIL command.
--
<a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>
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