Actually, subscribers have the option to "opt-out" so if you
know what you're doing and WANT to use another smtp
server/use port 25 for some reason you still can.
At a cost I imagine. CBF researching it.
reasonable compromise between, "disabling their
service" and being proactive about security in my personal
opinion. Not that blocking port 25 is going to be a magic
bullet against all viruses/worms etc, but a start anyway.