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I (Marc Perkel/Junk Email Filter) have now been using this technique for almost 2 years now without any problems. I am now harvesting the data and developing black lists based on hosts that connect ONLY to the highest numbered MX records and do not close the connection with the QUIT command after receiving a 4xx error. The blacklist has grown to over a million entries. The block list is public using our hostkarma list. Go to http://wiki.junkemailfilter.com/index.php/Spam_DNS_Lists fo SA rules to use this list.

Help Build a Blacklist

If you want to get rid of some spam using fake high MX records and you want to help us (us being junkemailfilter.com) build a blacklist then you can use our host (tarbaby.junkemailfilter.com) for your fake MX record. We will return a 451 error after the DATA command. This blacklist will help us and everyone using our blacklist to track virus infected spambots. And you are welcome to use our blacklists to further reduce spam. Your MX records can be as simple as this:

 mail.yourdomain.com  10
 tarbaby.junkemailfilter.com 30

Greylisting

Instead of a 2nd fake MX you can use greylisting, which returns a temporary "Come Back Later" error for users currently not known. It has the advantage of helping you on the primary MX directly, and rejects about 60% of the connections here. This is because spammers only try to send once, and if there is an error, they drop it. Real mail servers retry later.

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