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To remove a shared mailbox, go to Settings > Accounts. Then tap on the shared mailbox and select Delete Account.
Follow below steps to add shared mailbox contacts to the phone.
Open Outlook application on the mobile
Click on Apps at the bottom of the application and select Contacts.
Click on the Filter icon (Three small horizontal lines) on the top right of the screen.
Click Done after making sure the box next to Regius Mailbox is checked.
Now all the contacts should appear in the outlook mobile app.