Blocking ports one by one and filling the Internet by application level
proxies (SMTP gateways for port 25) is not a road worth travelling.
Blocking port 25 for dynamic ranges means they can't send email, so that
drone are pretty useless for spammers on that account. Trojan horses
would have to use local information for the user's own account (from
Outlook or such).
ISP's could then, I suppose, limit every user to 5 emails a minute (or
any other number).
That combined with domain-keys and sender-ID could make for a much
prettier Internet, don't you think?
Abuse using port 25 is a major issue today, why not solve it? If a user
wants it open, they could always ask for it or even pay more money.
Perhaps move to a static IP?