Ну или еще посмотреть man spamass-milter. Там тоже много чего интересного найдете,

. Например:
-i networks
Ignores messages if the originating IP is in the network(s)
listed. The message will be passed through without calling Spa-
mAssassin at all. networks is a comma-separated list, where each
element can be either an IP address (nnn.nnn.nnn.nnn), a CIDR
network (nnn.nnn.nnn.nnn/nn), or a network/netmask pair
(nnn.nnn.nnn.nnn/nnn.nnn.nnn.nnn). Multiple -i flags will append
to the list. For example, if you list all your internal net-
works, no outgoing emails will be filtered.