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

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Once that is done the portal should display the domain selected above in the portal list shown below
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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
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