No-listing: a cheap anti-spam strategy
I read recently about a modest configuration change that administrators can make in their DNS records to significantly reduce the amount of inbound spam. I wouldn't advise it for an enterprise without a lot of testing, but it certainly intrigued me.

Known as "No-listing," it is not well-known yet, but is very uncomplex. You simply add a dummy MX record as the highest priority MX in your DNS. Make sure the target host is one that does not accept traffic on port 25, but is still an IP controlled by you. Mail servers following proper RFC protocol will attempt delivery at the first MX record, and after failing, will move to the second, third, and so on. Apparently the bulk of the spam-mailers do not bother to try additional MX records, and just ignore domains whose primary MX record does not respond. One writer about "no-listing" claims that 90% of his inbound spam was abated using this method and that to date he has had no problems receiving valid mail. Read more about no-listing here.  ¶ 9:47 PM