High Quality Content by WIKIPEDIA articles! Nolisting is a method of defending electronic mail users against e-mail spam. The idea is that by having a non-existent primary mail server and a working secondary mail server, attempts to contact the primary mail server will always fail. If the mail is sent by a correctly configured email server, the sending server will then try to contact the secondary mail server, and should succeed. Spammers frequently use custom software that does not retry higher-priority MX records.