] Internet mail suffers from the fact that much unwanted
] mail is sent using spoofed addresses %u2013- "spoofed" in
] this case means the address is used without the
] permission of the domain owner. This document describes
] mechanisms by which a domain owner can publish its set of
] outgoing MTAs, and mechanisms by which SMTP servers can
] determine what email address is allegedly responsible for
] most proximately introducing a message into the Internet
] mail system, and whether that introduction is authorized
] by the owner of the domain contained in that email
] address.

Here's the internet draft. This is the merger of Microsoft's "Caller ID for email" and SPF.