> I use Greylisting [1], and it's been really effective. No false > positives (so far), and 0 to 2 spam messages a day. All this for a > mild ~15 minute delay on genuine emails (but only for the first time).
sites like plan9.bell-labs.com tend not resend email with prec. bulk even when given a 45x error. smtpd 451'd a couple messages within the last week due to dns errors. the sender did not retry. i assume that they would have not been resent regardless of the text of the 451 message. i did run greylisting for several days. i found it cut down on spam only about 20%. those bots are getting smarter. - erik
