Marc Merlin has developed a unique variant of greylisting for SpamAssasin+Exim. Details at: marc.merlins.org/linux/exim/files/sa-exim-cvs/README.greylisting, SA-Exim at: marc.merlins.org/linux/exim/sa.html and finally the SA-greylisting implementation here: marc.merlins.org/linux/exim/files/sa-exim-cvs/SA-greylisting.diff
