Steven M. Bellovin wrote: The spammers are already creating throw-away domains;

Indeed, a little stockpile has never hurt anybody; by registering them now they'll even have some that have been registered for 11 months when they use them in March 2005. There already are RHSBLs lookup shops that attempt to block these as well.

they'd do the same with mail sender authorizations.

True; although they will not suppress spam mail sender authorization schemes do have two advantages: a) they will curb some (from the dumber spammers that still send their crud on behalf of joe12345@bigeye_isp.com) and b) they will seriously reduce phishing schemes on behalf of ebay.com or mybank.com. Michel.