I would recommend setting up
support@foo.bar with a zimbraMailTransport pointing to the RT server.
zmprov ma
support@foo.bar zimbraMailTransport smtp:rt.domain.com:25
Then set up Postfix on rt.domain.com to pipe email in as required.
Just be sure that RT relays mail out through Zimbra, so that the Postfix instance on rt.bar.com doesn't think that domain.com isn't a local domain and encounters a failure when it delivers a mail to a local user.
Another option (which is what we do) is to set up
support@foo.bar as a distribution list, or an account with a forward, which forwards email to
support@support.domain.com. That way postfix on the RT machine will have its own subdomain and you don't have to deal with conflicts. You don't have to make other people aware of support.domain.com, but it will just be used behind the scenes to map support mail into RT.
Final option would be to have RT pull mail from Zimbra using POP3, also.