Ideal Webmail anti spam solution

please, can anyone with information assist on an ideal solution for anti spam in a webmail situation.

I have a proxy server, as my gateway in a commercial internet center. I would want to block spam mails from passing through my network directly(from LAN to internet) and indirectly(from a server on the internet that utilizes open ports on my LAN network). By webmail, I mean Yahoo, hotmail, gmail etc.

An application I could install on my proxy server/that has proxy capabilities…etc

thanks