Greylist is a spam blocking plug-in for Exchange 2000/2003 with some support for IIS. Should be installed on the Exchange server that faces the Internet and will then provide grey listing functionality. In most scenarios this will block 80-98% of all spam.
In default mode it will make use of a local MS Access database but it can also be configured to use a MS SQL database for a shared grey list database across the enterprise.
The application is released as freeware and is fully functional, but by registering more options are available like custom reply messages. It consists of two parts.
Greylist installs as a .dll and connects to the SMTP service. Greylist admin creates and configures Greylist as well as controls the settings and the white list.