If you (and others in your position) would do this (block outbound SMTP), you would be doing the Internet as a whole an immense service. As long as you make it an easy webform and advertise it well in advance of implementation, your customers shouldn't complain too much. You could proactively set your firewall to log all outbound SMTP transactions and send the administrative account for that address a warning that this will be blocked in X days unless they register.
[snip]
Now, asking at signup time if you'd like this protection, with the option to say "no leave me alone" or "help me stay clean, block these ports except to these addresses" I don't object to, but please don't suggest that home users aren't allowed basic connectivity - if I wanted to be treated like a child I'd go back to school.
This isn't about protecting users (per se). It's about keeping the ISP networks out of the blacklists and reducing network saturation from "illegal" proxy traffic.
The change wouldn't prohibit customers from being able to send email. If they choose to not use our outbound SMTP server (which provides relay for on-net customers, regardless of their domain name), they simply register a request to leave port 25 outbound open for them (inbound wouldn't be touched).
After thinking about it for a while, I don't see why it would be too much of a problem, as most dialup providers already block outbound SMTP except to their mail servers. Unfortunately for dialup accounts, you don't have the optionto use a different mail server except on a different port (which works really well for our customers' mobile users).
-- Bryan
