What does help is the record in reverse DNS (as in MTAMARK)
This ignores the reality of large ISPs, and all three network registries, of
being lazy in delegating and enforcing correct reverse DNS to begin with.
Ditto, re: RFC 2317.
(except SPF which at least tries to do things in more combined
way focusing how to best accomodate different client implementations).
I'm going to say this once about SPF. (Meng: Pay attention!) It's trying to
be all things to all people. In the end it will annoy everyone, that is if
it isn't too complex to be useful.
As for accomodating client implementations, that's an implementation problem
solved long ago with AUTH and SUBMIT and many other non-SMTP mail injection
protocols. There are political and corporate reasons, not technical, for
their lack of use.[1]
[1] ISP branding (aka: "greed") notably. Also corporate image, compliance
with certain federal legislation (PIPEDA in Canada) and so on.
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