This is probably old news to many, but I just stumbled across it.
http://www.circleid.com/article.php?id=1039_0_1_0_C/
Suresh writes:
And speaking of controversial...
> If the top 50 domains in the world who are sick of the spam
problem implemented the non-SPF ban, this would force every other
domain in the world to comply with SPF - unless they don't care
for their e-mails.
That is going to be far, far more ruinous, if only because SPF is
badly thought out and fails horribly in several edge cases. Several
spammers can, and do publish SPF records. And the implication of
publishing SPF records absolutely forces people to rely only on
their email provider's mailserver assuming the restrictive - all
SPF record - more conservative ?all and ~all records are not going
to be very useful, and -all is guaranteed to lose you mail, given
the number of forwarding email providers who don't implement the
other side of the SPF coin - ses/srs return path rewriting of
forwarded mail.
-wayne