RE: Open relays and open proxies

Block port 25 inbound and outbound, and setup a cluster of managed,
authenticated, secure, mail relays (both in and out) for your customers.
You're never going to get all the proxy ports, but regardless to send spam
they have to hit port 25 on the outbound.

I don't want to stir up old debates about SMTP filters on the edge, but I do
think it's a viable solution if you provide a simple to use alternative for
your userbase. (God knows I have had enough problems with sales staff
plugging laptops into foreign networks and trying to get back to our mail
boxes)