Have you full access permission with other mailbox?
If so, there's a feature in Outlook --- Auto Mapping.
If a user is granted Full Access permissions to another user's mailbox or to a shared mailbox, Outlook client will automatically loads this mailboxes into your Outlook profile.
However, this feature isn't available in OWA. If we need access to other's mailbox, we can click your mailbox, and "Open another Mailbox".
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