With the new Exchange 2010 RBAC model, one of the configuration changes is regards to EWS and Application Impersonation. Instead of defining the ACL’s directly, you configure roles for the appropriate permissions.
If your in a resource forest setup, things are a little different. Here are the steps.
Your service account, named ServiceAccount needs to be assigned Application Impersonation rights to all the accounts in the Accounting OU. The user accounts are in client.corp and the Exchange mailboxes are stored in exchange.corp and there is a forest trust between the two.
Create a Universal Security Group in client.corp named UG-ExchangeImpersation.
Create a new linked role group with the Application Impersonation rights bound to this group. Run the following from an Exchange Management Shell in exchange.corp:
|$remotecred = get-credential
Put in a user name of an admin account for client.corp
New-RoleGroup ROLEGROUP-ExchangeImpersonation –LinkedForeignGroup “UG-ExchangeImpersation” –LinkedDomainController DC01.client.corp –RecipientOrganizationalUnitScope ‘exchange.corp\Accounting’
Add serviceaccount to the UG-ExchangeImpersonation group in Client.corp.
Ensure that serviceaccount has a linked mailbox in exchange.corp.
Once AD replication finishes, you should have impersonation rights on all users in that Organizational Unit!