Hi Mike,
Thanks for reaching out to Microsoft Q&A.
You can access another person’s mailbox if you have been granted the appropriate permissions to their mailbox. Permissions can range from read-only to creation, deletion, and allowing responses to messages.
Chek details here - https://support.microsoft.com/en-us/office/share-and-access-another-person-s-mailbox-or-folder-in-outlook-a909ad30-e413-40b5-a487-0ea70b763081
Steps here - https://learn.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/how-to-access-other-mailboxes
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