Nolisting is one of the methods 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 thus, 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, which do not retry higher-priority MX records.
As the SMTP requires, email servers for any given domain must be provided in a prioritized list (namely,MX records). It also specifies further mandatory error handling behavior when servers in that list cannot be contacted. Nolisting involves purposely creating unreachable MX records, so that senders who have implemented this error handling code can deliver mail successfully.
This technique relies on spammers using custom software, which ignores the SMTP protocol. Hence, it is not a viable long-term solution. Spammers can cheat Nolisting by simply using standard email server software or by adding a little error-recovery to their custom software. Thankfully, Nolisting can be abandoned if it ceases to be useful.
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