To grant Impersonation Rights:
1. Open the Exchange 2007 Management Shell
2. Use the Add-ADPermission command in Windows PowerShell to grant the ms-Exch-EPI-Impersonation and ms-Exch-EPI-May-Impersonate permissions.
For example, if the Admin account is called userName, the commands are:
Get-ExchangeServer | Add-ADPermission –User “DOMAIN\userName” -extendedRights ms-Exch-EPI-Impersonation -InheritanceType none
Get-MailboxDatabase | Add-ADPermission -User “DOMAIN\userName” -extendedRights ms-Exch-EPI-May-Impersonate -InheritanceType none