This memo defines how to store in the Internet Domain Name System the mapping information needed by e-mail gateways and other tools to map RFC822 domain names into X.400 O/R names and vice versa. Mapping information can be managed in a distributed rather than a centralised way. Gateways located on Internet hosts can retrieve the mapping information querying the DNS instead of having fixed tables which need to be centrally updated and distributed. This memo is a joint effort of X400 operation working group (x400ops) and RARE Mail and Messaging working group (WG-MSG).