Running 7.0.2, we have a primary domain that houses our corporate users.
We have an external domain that connects to an Exchange system in our
sister company.

I'm getting ready to build a secondary domain to house some
non-corporate users and I'd like to limit their visibility to only the
secondary and primary domains but exclude their visibility from the
external domain. How can I do this? When I look at the visibility
field, my options are system, domain, PO, or none. None of these do
what I need to do.

Any ideas?