Most Postfix lookup tables are used to look up information.
Examples are address rewriting (the lookup string is the old address,
and the result is the new address) or access control (the lookup
string is the client, sender or recipient, and the result is an
action such as "reject").
With some tables, however, Postfix needs to know only if the
lookup key exists. The lookup result itself is not used. Examples
local_recipient_maps that determine what local recipients
Postfix accepts in mail from the network, the
that specifies what domains Postfix delivers locally, or the
mynetworks parameter that specifies the IP addresses of trusted
clients or client networks. Technically, these are lists, not
tables. Despite the difference, Postfix lists are described here
because they use the same underlying infrastructure as Postfix