Today a Customer asked me this Question. After placing a considerable amount of thaught I came up with this Answere

1) Based on my experience, we cannot prevent users from deleting emails from his own Inbox. The SELF account must have the Full Access permission on the mailbox.

As a workaround (not actually a workaround but another scenario to achieve this through reviewer permission), you may create another mailbox account (e.g. bob) for the user (e.g. administrator), and then grant bob Reviewer permission on both the mailbox level and Inbox level. Like below:

After that, bob can open the administrator’s Inbox in Outlook via File -> Open -> Other User’s Folder, and he can only view the emails.This seems to be the only way to achieve your specific objective.

Hope this is helpful for someone.