Admins occasionally need full access to another users mailbox (for example, to verify the mailbox is safe to delete once a user leaves the organization).
Always use Outlook delegation as the end-user to share a mailbox first! However, if you need to do this administratively, then you can do it via the Office365 administrative web UI.
- Log in the Exchange management console at office365.com. From the Dashboard, select “mailboxes” under the “recipients” heading.
- Find the user whose mailbox you need access to (for example, by searching for their name) and click the Edit button at the top of the user list
- Switch to the “mailbox delegation” tab on the left, then click the add button under the “Full Access” heading.
- A searchable list of users in the domain will open; select the person who needs access (normally, you) and then click OK/Save to return the Exchange console.
You now have full access to the mailbox and can add it to your Outlook. The easiest way to do that is to just restart Outlook, it should automatically add any mailboxes to which you have full access permissions to your folder list.