As the Office 365 admin, you may have company requirements to allow some users to read email in another user's mailbox. For example, you may want to enable an assistant to read email in their manager's mailbox.
Read email in another user's mailbox
Go to Office 365 admin center > Users > Active Users.
Click or tap on the user you want, and then click or tap on mailbox permissions.
In the search box under Read and manage email to this mailbox, type the name of the user or users that you want to allow to read email from this mailbox.
Once you've selected your users, click Save.
Want to know how to read email in another user's mailbox? Check out the following topics: