spam-filtering policy

Dear All,

Since there's no 100% effective spam filtering mechanisms
so far, is it 'effective' to block (every/some) domains/IPs
and have the admin of those domains/IPs  send an online
application to whitelist the domains/IPs?

If it is an effective policy, wouldn't this create more
of a hassle for the admins of valid/innocent domains?

If it isn't an effective policy, why does anyone use it?

The reason why I'm asking is that my co-worker has been
trying to send an e-mail to a customer whose ISP seems to
be using such a spam-filtering policy.  This ISP is blocking
my company's domain, for some stupid reason.  My company
certainly doesn't spam.

Since I'm ever-learning the ins-and-outs of mail server
administration, I'm curious as to what everyone here
thinks.

Thanks

Ed




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