Allow relaying for specified IP ranges.

[Sam]

It would be nice if we could configure the mynetworks setting in postfix.

This would allow us, for example, to allow relaying from docker containers (which mostly are on local, non-internet-routable) IP addresses.

http://www.postfix.org/postconf.5.html#mynetworks

[Huan Li]

Thanks, This make sense to me.

PR merged.