Give one user full control of another user’s mailbox in Office 365

To give yourself or another user full access to someone else’s mailbox in Office 365 you have to use powershell. It can’t be done online.

Mailbox access

In this example we are going to give Pieter full access to the mailbox of Danielle use this command:

Add-MailboxPermission  -User -AccessRights FullAccess

Inbox access

As an alternative you can also give permissions to an inbox only

Here’s the code:

Add-MailboxFolderPermission -Identity\Inbox -User -AccessRights Owner


For more detailed information, please refer below article:

Give an Administrator the Ability to Open and View the Contents of a User’s Mailbox( )

Open another user’s mailbox in Office 365

In Office 365 you can open another user’s mailbox. Here’s how.

Go to the outlook web access page. In the left column find your username and right click on it.

Right click on your username













Then select “Other user’s inbox”

Select other user's inbox
Select other user’s inbox












In the dialog box that appears, enter the name of the desired mailbox or use the addressbook icon to do a full search.

Enter the name of the desired mailbox
Enter the name of the desired mailbox






And click OK. Now you will see the other user’s mailbox appear in the left column.

There's your other user's mailbox
There’s your other user’s mailbox














When you receive an error stating that you have no permission to open the inbox when you are sure it exists, you may need to get the right permissions from your administrator.

Not found or no permission
Not found or no permission






I also have an article that explains how to set the right permissions.

Read it here.