Nolisting is a technique to defend electronic mail domain names against e-mail spam.
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.
[3] Some variants also suggest configuring the highest-numbered hosts to always return 4xx errors (i.e. "retry later").
[3] 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.