Right, this is what the SPF system does. Only the *last* relay can be
checked. The domain owner must authorize the use IP address of that
relay in order for the SPF system to give an OK. (It might still give
an "unknown" as well as a "denied".)
Ok, so if I'm the owner of a mailserver and I want to use spf, I have to
manually create a list of every IP address of every mailserver that I expect
to receive email from.
What if somebody sends mail from aol.com, and aol.com changes the IP address
of their mailserver?
What if somebody sends mail to my mailserver, and I didn't expect it to
come?
What if I'm an ISP, and my customers go make new friends so there was no
possible way I could have expected email coming from joeschmoe.com, or
whatever?
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