I think you are right certainly for the time being. Open VZ standard which will run on machines with up to 4Gb RAM has a limit on the virtula machine
RAM size of 700 and something Mb.
The enterprise version which will run if you have more than 4Gb RAM will allocate over 3Gb to any virtual server if it is avaible.
Hence Zimbra is not getting enough RAM for some reason and the MTA stops.
So use Enterprise Open VZ
In the long run it would be nice to pick ready configured operating system templates off the peg.