Virus infected spam bots ignore requirements that the email start at the lowest numbered MX record and move up the list if a failure occurs. They try the highest numbered MX records first thinking that the backup servers have less spam filtering than the low numbered MX servers. Spam bots usually do not retry on failure but move on to the next email address in their list. Thus adding fake high numbered MX records is an effective way to reduce incoming spam.
One can also reduce spam but having a fake lowest MX record as well. This causes real email to have to retry as well but it only adds a second to the delivery time. Some people report as much as 90% reduction in spam bot spam using this method.
mx1.example.com - 10 - dead IP
mx2.example.com - 20 - real server
mx3.example.com - 30 - dead IP
