Give mailbox permissions to another user in Office 365

As the Office 365 admin, you may have company requirements to allow some users access to another user's mailbox. For example, you may want to enable an assistant to send or read email from their manager's mailbox.

How to gain full access to a users mailbox in Office 365 as an Admin
  1. Open a web browser and log into admin.microsoft.com
  2. Select Exchange on the left side under Admin centers.
  3. Click mailboxes under recipients.
  4. Now double click the user you would like to edit permissions for.
  5. A box will appear and click mailbox delegation on the left.
  6. Here you can give users 3 different permissions on the mailbox.

    Send as: Allows a delegate to send email from the mailbox as the mailbox owner.
    Send on Behalf: Will indicate that the message was sent by the delegate on behalf of the owner.
    Full access: Allows a delegate to open the mailbox and behave as the owner.
  7. Select the permissions and the users then click save.
Note: you can also edit permissions for shared mailboxes.

To gain access to the folder online

  1. Click the waffle on the top left and select Outlook.

  2. Now click the profile button on the top right and select Open another mailbox...

  3. Type in the user and click Open.

  4. The mailbox should also auto map to your Outlook profile on your desktop. If you have a profile set up, open Outlook.

  5. You should see the mailbox in your list of mailboxes on the left hand side. You can now view the mail within Outlook.

  6. To send as that user, open a new message.

  7. Click options.

  8. Click From under Show Fields.

  9. Now click the from drop down and click from again.

  10. Select the user you would like to send as and click OK.

 

Source: https://support.microsoft.com/en-us/help/10048/accessing-other-people-s-mailboxes-in-office-365