> Paul, I appreciate what you're saying above.  But isn't it likely that if
spammers
> are going to the trouble to add SPF records that they're also going to the
trouble
> to retry after a tempfail, and thus defeat greylisting?

I would disagree with this.

Spammers don't have to be the authors of spam-sending software.  The program
authors would care (or not) about retransmitting after a tempfail.  The
people who obtain and USE the software to send spam will control the DNS
domains they set up (BuyMyStuff.com, etc.) will control whether they want
SPF records in those domains.

