Because you probably won't continue to block their messages, once they
(for example) start spewing through the zombie armies.
It is completely illogical to block mail from a provider because
they have a zombie on their net. There is not a single ISP in this
country who could guarantee that they did not have any.
The way to deal with zombies is to have a better response protocol
to put ISPs on notice that they have a problem with a particular
address. Most ISPs do not want to be hosting zombies on their net
and they will kill them the moment they find out about them. That
is if the comunication reaches the right part of the company.
Blacklisting zombies is certainly legit. But blacklisting every other
user of that ISP is not.
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