The 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:

O AliasFile=null:

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.