I would suggest instead that you have mandatory sending via your relays, and allow inbound connections to port 25.
We're a fairly big provider on the GRIC (global roaming) network. That means that it's not feasible for us to prevent many of our POPs' users from contacting off-net SMTP servers. Running an enforced SMTP service via transparent proxying wouldn't stop the spam problem, it would just shift it and probably get the proxy system black-listed as an open-relay's relay. Anyway, like I said, we don't *have* a spam problem on our dialups. By virtue of our filters we don't have any open relays on dialup. ADSL is a different matter and we do have occasional problems with open relays and/or worms there. Unfortunately the UK incumbent wholesaler(*) doesn't provide a way to filter ADSL traffic within their ATM core. The only way to do it is to put another router between our network and the "BT Central" router that connects their ATM cloud to us. Of course that doesn't provide any inter-customer filtering, since that traffic never reaches our network :( Ray (*) BT - they have a nearly complete monopoly on the local loop.