Nolisting
Nolisting is the name given to a technique to defend electronic mail domain names against e-mail spam. Each domain name on the internet has a series of one or more MX records specifying mail servers responsible for accepting email messages on behalf of that domain, each with a preference. Nolisting is simply the adding of an MX record pointing to a non-existent server as the "primary" (i.e. that with the lowest weighted value) - which means that an initial mail contact will always fail. Many spam sources don't retry on failure, so the spammer will move on  to the next victim - while legitimate email servers should retry the next higher numbered MX, and normal email will be delivered with only a small delay. Implementation. A simple example of MX records that demonstrate the technique: This defeats spam programs that only connect to the highest priority (lowest numbered) MX and do not follow the standard error-handling of retrying the next priority MX. Similar techniques. There are alternate techniques that suggest "sandwiching" the valid MX records between non-responsive ones. Some variants also suggest configuring the highest-numbered hosts to always return 4xx errors (i.e. "retry later"). A  simple example of MX records that demonstrate the technique: Greylisting also relies on the fact that spammers often use custom software which will not persevere to deliver a message in the correct RFC-compliant way.