I suspect that the majority of email sent has three of those four
implemented. If not quite a majority it's _definitely_ a large fraction.
(Yes, I'm cheating. Nobody other than the very, very gullible
implements SenderID as such, but a valid SPF record is also a valid
SenderID record).
Since spam is said to be 90%, 95%, 99% of all email, but you say most is
sent with sender id. Are you suggesting most spam is sent with valid
sender id? Don't think so.
Having an SPF record is not the same as implementing SPF unless you
block all email that arrives that is not authenticated through SPF. You
are just posting a sign saying you'd like to comply, but don't actually
implement it on the receiving end. And if you block everything that
doesn't have SPF configured, you'll lost much of your legit email.
David
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