By default, Postfix will not relay mail that did not originate from the local domain. This means that if a Postfix installation running at hudson.org receives mail intended for hudzilla.org, and that mail did not originate from hudson.org, 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 so:
mynetworks = 192.168.2.0/24, 10.0.0.2/24, 127.0.0.0/8
The IP address needs to be specified in CIDR format. For a handy calculator, head on over to http://www.subnet-calculator7cidr.php. You must restart Postfix for this change to take effect.
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