it is acceptable for a lookup to fail because of a temporary failure
of a system resource. When reading from a network database map (such
as the DNS name server), temporary failures (such as server down)
generally cause email to be requeued for a later try. However, you
might sometimes find it desirable for a database
map's temporary failures to be ignored. In such
cases, you can enable this -t database switch.
With it set, a temporary error will cause the mail to be delivered.
Note that failure of a key to be found in a database map is not a
temporary error. Also note that this switch just determines the
outcome of a message. It does not affect the nature of the returned
value. To affect the return value on temporary failures, use the
-T database switch (-T).