Google: Manage Other User's Mailbox as A Google Apps Administrator

As a Google Apps Administrator, you can only manage or view other user's mailbox with the following methods:
1.  Reset the account's password
2.  Delegate the user's account to the Google Apps Administrator. However, this method requires the authorization or permission from the respective user
3.  Use Google Vault to search all the emails ( Inbox, Sent and Trash )