Spey

a greylisting SMTP proxy

Spey is a simple SMTP proxy that allows you to easily add greylisting (and other anti-spam measures) to your existing mail server. It's highly effective, highly efficient, and because it rejects spam before it's been transmitted, can significantly reduce the load and expense of dealing with spam.

Spey Sourceforge page

Spey is hosted on Sourceforge. See the page there for more information.