DNS-based White/Black/Grey-list

pyGreylist is a milter plugin written in Python that implements the greylist filtering method proposed by Evan Harris. It works by assuming that, spam engines will not retry sending their junk mail on a temporary error. The idea is to reject mail on a first attempt and then accept it after some time has elapsed. According to the SMTP specification, properly configured mail server should attempt to retry a delivery if it receives an temporary failure code.