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Paul Arnold kindly contributed a Cisco ACL for blocking smtp traffic from all the Chinese and Korean net blocks listed on this site. I have since mofified and continue to modify Paul's original contribution (made on 05.29.02) to reflect subsequent adds, deletions, and modifications.

Here are Paul's notes and instructions for this ACL.
The current rules just block SMTP, but to make it more restrictive you can:
1. search and replace "tcp" with "ip" (this will include tcp, udp, and icmp traffic)
2. search and replace "eq smtp" with "" - this will make the rule block ALL ports from the source as opposed to just port 25.

The instructions for the file are:
1. replace the text "yyy" with the number for your incoming access-list.
2. replace the text "zzz.zzz.zzz.zzz" with the ip address of the SMTP server.
3. add the access list to your router.

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