Internet
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... The purpose of this memo is to explain how mailers are to decide how
to route a message addressed to a given Internet domain name. This
involves a discussion ...
... such as for certain UUCP and CSNET hosts which were not directly
attached to the Internet, but these hosts could be handled as special
cases in configuration files ...
... Finally, it should be noted that this is a standard for routing on
the Internet. Mailers serving hosts which lie on multiple networks
...
... domain system to help them
decide. However, once a mailer decides to deliver a message via the
Internet it must apply these rules to route the message.
...