One other thing (feel free to email me off list), what is the difference between Sender ID and using SPF records? Or are they the same thing.... I like SPF, I like the fact that you control it within your own domain via your DNS server. When I hear Microsoft talk about Sender ID I get nervous, I envision some type of paid subscription to be listed on some central repository that Microsoft controls!
I'm not sure of the current status, since I stopped following it last fall, but initially "Sender-ID" was the name given to a superset of SPF and Microsoft's Caller-ID scheme. Caller-ID also used DNS TXT records, but the syntax was XML-based instead of fieldname:data style. It also checked different headers than SPF. Sender-Id would check either syntax. (I don't remember what it was supposed to do if it found both.)
Sender-Id looked promising, as it combined two very similar plans, one of which had already been deployed by quite a few early adopters...but Microsoft wasn't willing to license their parts freely enough to satisfy -- well, much of anyone -- and the IETF group working on it shut down without settling on a standard.
SPF has a page on Sender-ID basics, including the PRA header that Mark mentioned -- at http://spf.pobox.com/senderid.html
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