] Standard SMTP email is anonymous and forgeable.
]
] SPF closes this loophole. SFP is primarily an
] anti-forgery effort. Any benefits in the area of reduced
] spam, worms, viruses, etc are pleasant side-effects. That
] said, if SPF causes spammers to send mail from their own
] domains, we'll be better able to identify and block those
] domains.

Another flavor of sender-authentication. This is a superset of
both Reverse MX and the Designated Mailer Protocol. This is pretty
easy to deploy both sender and receiver side.

SMTP Sender Policy Framework