- Null MX
The Null MX proposal is a way of saying that "this
domain name never acceptse-mail " viaMX record s. It is specified in the now expiredinternet 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 theroot 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 DNSEXTworking 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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