sendmail, 4th Edition

bestmx

Look up the best MX record for a host V8.7 and later

The bestmx database-map type looks up a hostname as the key and returns the current, single best MX record as the value. Because bestmx is a type, not a database map, you need to declare it with a K configuration command before you can use it:

Kbestmx bestmx

One use for this database-map type might be to see whether a particular host has any usable MX records:^[344]

Kbestmx bestmx
...
R $*< @ $+ > $*                $: $1<@$2>$3 < $(bestmx $2 $: NO $) >
R $*< @ $+ > $* < NO >         $#smtp  $@ $2 $: $1 < @ $2 > $3
R $*< @ $+ > $* < $* >         $: $1<@ $[ $2 $] > $3

In the first rule, we look up the host part of an address (which has already been focused by the canonify rule set 3) with the bestmx database map. The result of the lookup is surrounded with angle brackets and appended to the original address. The second rule looks for the NO caused by an unsuccessful lookup (the $:). The original address is then sent with the smtp delivery agent. If the hostname inside the appended angle braces is not NO, the host part of the original address is canonicalized with the $[ and $] operators.

bestmx is a special internal type that can utilize only a few of the K command switches, as listed in Table 23-8.

Table 23-8. The bestmx database-map type K command switches

Switch	§	Description
-a	-a on page 887	Append tag on successful match.
-D	-D on page 887	Don’t use this database map if DeliveryMode=defer.
-m	-m on page 888	Suppress replacement on match.
-q