Null MX

Null MX

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 [http://tools.ietf.org/html/draft-delany-nullmx 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.

Criticisms

Since, by default, this proposal causes additional load on the root name servers, it has not been well received on the IETF DNSEXT working group [ [http://www.ops.ietf.org/lists/namedroppers/namedroppers.2005/msg00939.html draft-delany-nullmx-00.txt good? bad? ugly?] ]

References


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