[...] real protection against zombie is not that simple, ISPs would
need to have additional equipment that will redirect all traffic from
port 25 to their own relay servers
No, it would be enough to block port 25 connections except to their
user-submission hosts. (Many providers do this already.)
They may _prefer_ to redirect attempts to connect elsewhere, but that's
a different issue.
Most of the rest of what you say I agree with, including the basic
point that
ISP steps to deal with zombies is not a simple process and can be
quite expensive for ISPs.
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