On that note, is there zero spam from ISP servers? I'd suggest not.
Greylisting is meant to cause spam sent from non-MTAs (i.e. drones) to get deferred; they don't tend to have mechanisms to deal with 400 series deferrals.
spamhaus has just introduced zen. and people using sbl-xbl.spamhaus.org for RBL's might want to change to zen.spamhaus.org instead See: http://www.spamhaus.org/zen/ Zen adds the ability for ISP's to add to the PBL to list ranges which shouldn't send email out or for example dialup ranges which have a policy of no servers (ie email). I've used this for a short while now with no complaints but its reduced the amount of spam from bots quite a bit with the inclusion of the PBL in compared to the old sbl-xbl Thanks Craig http://www.spam.co.nz