Frank Ellermann wrote:
And the last case, that's what we are talking about here, is
a non-local alias, where the receiver changes RCPT TO a 3rd
party, and forwards the mail across a *second* border, again
coupled with query=mx etc. as at the first border. Because
this is a third party it can normally check SPF again, and
for an unmodified MAIL FROM this either FAILs, or at least
it can't PASS.
That's what Frank softly called "the odd 5.3.6(a) loophole",
after rfc 1123 section numbering; see
http://www.gossamer-threads.com/lists/spf/discuss/32901#32901
For terminology, perhaps we may call it the odd _SMTP_ loophole,
since rfc 2821 still encourages vanilla forwarding (although it
also says that "any further (forwarding, gateway, or relay)
systems MAY remove the return path and rebuild the MAIL command
as needed".)
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