Rejecting port 25 except from IP range

[JoeRT]

One of my clients use Postini for mail filtering. It works great. But a number of spammers are going around Postini and sending mail directly through our FQ e-mail server. On dedicated servers I've blocked port 25 connections from anyone except the Postini servers and it works perfect. Does anyone know of a filtering script or processor that would do the same on shared hosting?

Thanks!