Nolisting

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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 send an email to the primary mail server will always fail. If the mail is legitimate, the originating server will then try to send it to the secondary mail server, and should succeed. If the mail is from a spammer, it will probably not be retried. However, spammers are known to bypass the primary mail server anyway and go directly to the secondary mail server (in violation of RFC 2821). This is because secondary mail servers often have much looser spam filters and security checks and spam that would get blocked by the primary often is accepted by the secondary. To defend from that one can set up a low priority fake server that always gives a temporary error.

A related technique is to include non-existent mail servers as both the first and last MX record.

The records would be configured as such:

MX 10 dummy-a.domain.example

MX 20 real-server-1.domain.example

MX 30 real-server-2.domain.example

MX 50 dummy-b.domain.example

This defeats spam programs that always connect to the lowest-priority MX.

Downsides to this technique include increased traffic from those spam programs that send to all MX records listed, and the danger of unknowingly losing mail from an improperly configured mail transfer agent or script.