An administrator can limit the domains visible to end user accessing a portal
In order to do that go to domain configuration and select portals the domain should have access to

Once that is done the portal should display the domain selected above in the portal list shown below
After the above changes are applied the portal should only display domains specified in the steps above.

Note: As one can see LocalDomain is not displayed as one of the domains in OWA portal as OWA portal was not selected under the LocalDomain accessible portals