milter-greylist is a stand-alone milter written in C that implements
the greylist filtering method, as proposed by Evan Harris.

Setup instructions:
http://www.howtogeek.com/50948/how-to-setup-milter-greylist-spam-blocking-in-sendmail/

Additional build options:
- SPF=ON - enables support for SPF using libspf2
- RBL=ON - enables RBL support, needs bind installed
- GEOIP=ON - enables GeoIP2 support using libmaxminddb
- LIBCURL=ON - enables URL checks support
