sendmail, 4th Edition

Caching MX Records

Although you are not required to have MX records for all hosts, there is a good reason to consider doing so. To illustrate, consider the following host that has only an A record:

hostB           IN  A   123.45.67.8

When V8.12 and above sendmail first look up this host, they ask the name server for that host’s MX records. Because there are none, that request comes back empty. The sendmail program must then make a second lookup for the IP address.

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 each and 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 ...