David Woodhouse wrote:
some people advocate against SPF because of the way it tries to
retroactively declare 20 years of SMTP behaviour as 'wrong'
Mostly people using some vanity "alumni" forwarder. And RFC 821
never permitted to keep a MAIL FROM as is in *any* relaying, let
alone in forwarding to third parties. RFC 821 had precise ideas
about forwarding, 251 "will do, but please use new address", and
551 "won't do, please try new address".
the fact that it causes genuine¹ mail to be thrown away.
Nope. In fact SOFTFAIL is more dangerous than FAIL depending on
the *combined* receiver policies. If the MX stamps the mail as
"suspicious", something behind the MX moves "suspicious" to a
corresponding folder, and if "suspicious" normally is spam, then
the user might let the mail rot until it's automatically purged.
OTOH a FAIL is hopefully rejected by the border MTA (accepting
FAIL at the border would be too stupid to discuss it, but it's
of course possible to identify FAIL too late = after SMTP), and
unlike a poor "forwarding" user who might have no idea what SPF
is, the originator is supposed to know how SPF FAIL can result
in "genuine bounces" (for your definition of "genuine").
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