Server 1 (Postfix) - 10.20.30.40
$ vi /etc/postfix/main.cf # add: relayhost = 50.60.70.80
$ service postfix reload
Server 2 (Postfix) - 50.60.70.80
$ vi /etc/postfix/main.cf # update: mynetworks = 127.0.0.1/8, 10.20.30.40/32
$ service postfix reload