I have a working Zimbra 8.0.6 OSE that is currently accepting mail from the outside to any of its locally defined domains and will also relay every mail from authenticated SMTP users.
There are other, external, MX servers that will receive, filter and then relay mail to Zimbra via SMTP: I'd like to configure Zimbra to not accept mail for local domains from everything but those MXes, and of course still accept and relay mail from any authenticated SMTP user. I went through the configuration, and documentation, and google, but did not find anything. I'm suspecting that Zimbra's configuration scheme / UI / whatever does not allow such a setup, but I would be glad if anyone could prove me wrong.