Hi @John B
This can be done by delegating access to a mailbox. If this is required, you can do this through the Microsoft 365 Admin Center. As a delegate, they do not need to authenticate to the mailbox with any password/MFA.
How do I access the mailbox (Enterprise, Midsize, Education)
After you grant delegation to someone (for another person's mailbox) -- it will show up in their Outlook as another mailbox (in Outlook or OWA).
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