When pre-V8.12 sendmail first looks up this host, it asks the local name server for all
records. Because there is only an A record, that is all it gets. But note that asking for
any record causes the local name server to cache the information.
The next time sendmail looks up this same host, the local name server will return the
A record from its cache. This is faster and reduces Internet traffic. The cached information
is ???nonauthoritative??? (because it is a copy) and includes no MX records
(because there are none).
When pre-V8.12 sendmail gets a nonauthoritative reply that lacks MX records, it is
forced to do another DNS lookup. This time, it specifically asks for MX records. In
this case there are none, so it gets none.
Because hostB lacks an MX record, sendmail performs a DNS lookup eachand every
time mail is sent to that host. If hostB were a major mail-receiving site, its lack of an
MX record would cause many sendmail programs, all over the world, to waste network
bandwidth with useless DNS lookups.
We strongly recommend that every host on the Internet have at least one MX record.
As a minimum, it can simply point to itself with a low cost:
hostB IN A 123.45.67.8
IN MX 1 hostB
This will not change how mail is routed to hostB but will reduce the number of DNS
lookups required.
9.3.8 Ambiguous MX Records
RFC974 leaves the treatment of ambiguous MX records to the implementor??™s discretion.
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