Null MX

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The Null MX proposal is a way of saying that "this domain name never accepts e-mail" via MX records. It is specified in the now expired internet draft draft-delany-nullmx from 2005.

The proposal uses an MX record in the DNS with a priority of zero and a host name of ".", such as:

example.com.      MX 0 .

When an SMTP server attempts to send mail to a domain with a Null MX record, the server will find that the domain name supposed to handle e-mail is ".", which is the root name server. Servers will then attempt to look up the IP address for the root name server and then connect to a mail server. As of 2008, there are currently no A nor AAAA records for the root name servers so the delivery will fail. E-mail servers that follow this specification can short-circuit this lookup and stop the delivery immediately; however, that is making the assumption that the root name servers will never have A/AAAA records, something that has not been guaranteed.

Note that a "this domain can not receive e-mail" policy is not the same as policies that can be specified by DomainKeys and SPF that say "this domain does not send e-mail". Some systems will reject e-mail from systems that can not receive e-mail back, but many do not.