The problem with that idea: it relies on ISP's distinguishing end users and mail servers. Some ISPs are known to make a distinction on price (i.e. they would charge much more for full access than not) or - as previous discussions have shown - do not even distinguish static ip and dynamic ip customers
Mark Perkel wrote: Imagine a policy where ISPs blocked port 25 for consumers by default and forced them to talk to mail servers on port 587 to send SMTP. Suppose that all SMTP servers who took email from consumers had port 587 open as well as port 25. If port 25 were blocked from consumers and they were forced to talk to servers on port 587, even without authentication, then a server could distinguish consumers from other servers. I think this kind of configuration could be used to help isolate virus infected computers from spamming and spreading.
