Frank Ellermann writes:
The receiver can run into these issues. The sender can't,
most FAIL policies by definition eliminate 251-forwarding,
because that was broken by RfC 1123. FAIL replaces it by
a 551-emulation, good riddance.
Exactly what in RFC 1123 "broke" 251-forwarding?
251-forwarding is extremely rare anyway. The more important issue is
that SPF FAIL breaks the much-more-common alias forwarding described
in RFC 1123 section 5.3.6(a). SRS works around that breakage by
deviating from how 1123 says to do alias forwarding. Hopefully most
people, even those who consider SRS a botch, would consider the need
to get legit email through to be more important than strict adherence
to 1123's 16-year-old description.
--
Dick St.Peters, stpeters(_at_)NetHeaven(_dot_)com