Grey list is a freeware add-on for Exchange that implements grey listing. It installs as .dll onto the existing SMTP service and runs against a local MS Access database or a (preferred) MS SQL 2000/2005 server. When using it with MS SQL then it supports database sharing across several Exchange servers that face the internet to ensure that the grey list is shared between the servers. It has been developed with the common Exchange administrator in mind and is available as freeware together with the source code.