null database-map type is an internal service that
always returns a failed lookup.
Normally, the null type is used internally only.
It can, however, be useful when used to replace another database-map
type so that it can force failure without causing an error. Consider,
for example, a tiny configuration file that does not need the use of
the aliases facilities. One way to declare
aliases would be like this:
This tells sendmail to use the
null type for looking up aliases. Therefore, no
aliases will ever be found.
None of the K command switches can be used with
the null database-map type. If you try to use any,
they will be silently ignored. No debugging switch is available to
watch this null database-map type.