+.I relayhosts
+Allowed relay clients. Each line is a host-suffix pair, separated by a
+colon. If the client's hostname matches one of the hostnames in the
+file, that client is permitted to send mail to any host (i.e., to use us
+as a relay), and the corresponding suffix is appended to all recipient
+addresses generated by the client.
+
+.I relayhosts
+may include wildcards:
+
+.EX
+ heaven.af.mil:
+ .heaven.af.mil:
+ hell.irs.gov:.irs.virtdomain
+.EE
+
+For historical reasons, the
+.B RELAYCLIENT
+environment variable overrides this table. If
+.B RELAYCLIENT
+is set, it has the same effect as there being a matching entry in the
+.I relayhosts
+file, using the value of
+.B RELAYCLIENT
+as the suffix.
+.TP 5