How do they prevent it?
Todd Holt
Actually, in my line of work, I come across a great many ISPs who block inbound port 25 to their customers to prevent them from becoming spam relays. I can even name a few who block outbound 25 as well - you can only send mail if it goes through their SMTP servers. For residential services, I actually tend to think this makes a lot of sense (even though I'm generally anti-port blocking).
Business class/level services that are designed for people to run servers from, however, should let you change the reverse DNS. I don't know that issue seem to be down there - here in Toronto we've got a few DSL lines from different companies that are 5Mbps down and 1Mbps up. Two of them gave us a /24 with the reverse DNS delegated to our servers, and the third gave us a /27 and faxed us a form asking what the reverse DNS should be for each IP. We wanted a few changed, e-mailed them, and it was done a few hours later. Oh, and these lines are each less than the equivalent of about $300US (one of them is significantly less).
If you're trying to run a mail server off of a service that isn't designed for it, well, that's your choice. But as AOL has shown, there are some unpleasant consequences.
Bryan
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