Domain Name System
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... hosts on its path from sender to ultimate recipient.
The Mail eXchanger mechanisms of the domain name system [22, 27] (and
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In general, the availability of Mail eXchanger records in the domain
name system [22, 27] makes the use of explicit source routes ...
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Sometimes a host is not known to the domain name system and
communication (and, in particular, communication to report and repair
the error) is blocked. To bypass ...
... contain only a mailbox and domain name, relying on the domain name
system to supply routing information if required. The use of source
routes is deprecated; while servers MUST be prepared to receive and
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... source routes in regular mail traffic was eliminated by the
introduction of the domain name system "MX" record and the last
significant justification for them was eliminated by the
introduction, in RFC 1123std3 ...