As a temporary measure for transitioning from my old mail server, I want Zimbra to act as the smtp relay for the old server. I think in Postfix you just setup the "mynetworks" in main.cf:
mynetworks = xxx.xxx.xxx.xxx/24
or use one of the mynetworks_style choices.
But I have already been caught with DNS issues with my Zimbra install, so I want to make sure that setting mynetworks won't screw anything up.
( By default mynetworks_style = subnet, and I would be changing that, I think)