Accessing another persons Mail Account

Access the persons mailbox who want's to give access to their own mailbox to someone else and go to:

Select  - 'Tools'
           -'Options'
           -'Delegates'

Then select the persons name who you want to have access to your mailbox from the global address list.
Make sure you select the correct access rights you want the person to have.
It may take 15/20 mins for the access to be granted.