I was asked the following question recently:  What are the minimum permissions necessary I need to grant a user in order for that user to be able to access the data in another user’s mailbox?

Automatically I referenced following article 821897 that states Send As and Receive As permissions are necessary.  What we came to find out was that the Receive As permission was the only permission necessary to access information in the mailbox and the Send As permission was only needed to add data into the mailbox and not simply access the information. 

After some research I have the answer to the question:

There are two methods to grant permission for a user to access another user’s mailbox through Outlook by selecting File -> Open -> Other User’s Folder.  If it is a custom application that is accessing the mailbox using WebDav or CDO code yet another set of permissions are needed. 

Source: http://msexchangeteam.com/archive/2006/01/25/418099.aspx