Thanks for the help, I can sort it out I think
Installing Zimbra is pretty straightforward compared to building a bunch of Postfix boxes from scratch, which is my usual method.
I suppose most people trying out zimbra who aren't "power users" would opt for @domain.com, I think the installer options could be scary for new people who may upgrade to paid version if they have a good experience. Explaining DNS at all to new users is tricky, lots of blank stares when you try to explain, Then you wind up ssh'ing in and doing it for them
Just one data point.