dgrey is a greylisting policy server that allows greylist information to be synchronized between multiple e-mail servers. All information is shared between the nodes and the nodes are equal - there is no master.
The nodes may lose communication for a while, and still be able to correctly resynchronize later. This works because all nodes keep a log of changes to the database. All communication is done across TCP connections, which can optionally be password protected.
dgrey works with mail transfer agents such as Postfix and Exim.
dgrey is written in Perl by Oskar Liljeblad. It is partially based on and heavily inspired by Postgrey by David Schweikert.