Greylite is not a patch for the mail server but resides in the Qmail chain. Greylite requires an Sqlite 3 database and Qmail running on the Tcpserver super server which listens for incoming connections and understand access restrictions like Tcpwrapper.

Greylisting involves denying all initial delivery attempts. Regular MTAs will attempt to redeliver, spammers and wormers will typically not, and this makes greylisting a useful technique for fighting spam. To avoid unnecessary delays to legitimate mail, Greylite uses a variant on this techniqe: servers that have passed the test once before are added to a whitelist and can deliver mail without restrictions in future.

The C source code of the program is available from the project homepage under the revised BSD license. In the few days since version 1.0 was released, the developers have made a number of improvements: Greylite no longer waits infinitely if the other end fails to respond but removes obsolete requests from the database.