By default, Postfix will not relay mail that did not originate from the local domain. This means that if a Postfix installation running at
gracehopper.net receives mail intended for
seymourcray.net, and that mail did not originate from
gracehopper.net, the mail will be rejected and will not be relayed. If you want to allow selected domains to relay through you, add an entry for the domain to the
main.cf file like this:
mynetworks = 192.168.2.0/24, 10.0.0.2/24, 127.0.0.0/8
The IP address needs to be specified in classless inter-domain routing (CIDR) format. For a handy calculator, head on over to www.subnet-calculator.com/cidr.php. You must restart Postfix for this change to take effect.