Here’s a quick way to give Administrator accounts full access to all users mailboxes in your Office 365 environment.

Create a Security Group which will contain your Admin accounts

• Log in to the Microsoft Office 365 Portal (https://portal.office.com/).
• Click Distribution Groups under Manage Outlook and Exchange Settings or simply under Exchange --> Recipients.
• Click the New button.
• Type a name and alias for your group, something like Tenant_Admins.
• Tick the box that says Make this group a security group.
• Add your tenant administrators (or people who you want to have access to all users mailboxes) as members of this group.
• Save the group.

It might be worth hiding this group from the Address Book so your administrators don’t get hassled with emails from your users.

You can do that by double clicking the group and ticking the Hide this group from the shared address book box.

Grant this group Full Access permissions to all users mailboxes

Now we need to give this group full access to all users mailboxes. We need to do this in PowerShell. The cmdlet below will give all members of the group we created above full access to all User Mailboxes.

Firstly, connect to Office 365 using PowerShell as an administrator.

Now, run the following cmdlet. But remember to replace the bold bit with the security group you created above (you can even specify a single administrator account, not only security groups).

Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox')} | Add-MailboxPermission -User [email protected] -AccessRights FullAccess -InheritanceType all

So what if we want to remove these permissions?

Easy, just change the second cmdlet from Add-MailboxPermission to Remove-MailboxPermission:

Important things to remember

If you specify a security groupn and not a single administrator account, all the users inside the Tenant Admins will now have access to look inside all your users mailboxes.

You will need to re-run the first cmdlet each time you add a new mailbox to make sure that the permissions apply.