How to assign permissions to another user's mailbox through Office 365 Admin Center

For example, we want USER1 to have reviewer permissions to USER2's Mailbox+Subfolders. I know that you can add USER1 to USER2's mailbox delegation in the Exchange Admin Center, but that just gives the option for Full Access and Send As/Behalf permissions. We just want USER1 to see all folders/emails (reviewer).
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Vasil Michev (MVP)Commented:
There's no such thing as Reviewer permissions on the entire mailbox. You will have to adjust them on per-folder level, and you need to do it via PowerShell (or have the user do it from his Outlook).

Here's an example script to achieve this:

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Health Payment SystemsAuthor Commented:
Thank you for the information.
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