Exchange 2007 and 2010 differ in how to configure Impersonation to allow users to send as other users via EWS.
This link is the 2007 way:
http://msdn.microsoft.com/en-us/library/exchange/bb204095(v=exchg.80).aspx
This link is the 2010 way:
http://msdn.microsoft.com/library/bb204095.aspx

The user to give impersonation rights to is the user id the application or service is being run with, not the primary mailbox used to connect to the Exchange service.