I think you miss a key point....
SPF is designed to work at the domain level for domain owners. It offers
the exists: mechanism for more finely grained policies when necessary, but
this requires a custom DNS back end that is complex to implement correctly.
Pobox.com had a beta of this, but no one, AFAIK, has gotten this into
production. This is a hard engineering problem (I'll be glad to say I was
wrong when someone shows running code).
IIRC, we were talking about cheap providers. Cheap providers aren't going
to do this.
From reading your last message it isn't clear to me that you understand the
difference between REJECT and BOUNCE. This is essential.
10 years ago open relays were the norm.
5 years ago bouncing was standard.
Today bouncing is considered anti-social at best. Forwarding to much spam
will get a forwarder blacklisted.
Even if your analysis was correct and your proposal would not result in
significantly more bounces, the trends are against you.
No bounces except to a verified mail from is the best practice today.
Any new proposal really has to have essentially zero backscatter risk.
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