Configure Access to Entire Exchange 2007 Mailbox Database

Source URL:
http://technet.microsoft.com/en-us/library/bb310792%28EXCHG.80%29.aspx

* In the Exchange Management Shell, use the following command to
allow access to all mailboxes on a given mailbox store:
Add-ADPermission -identity "mailbox database" -user
"serviceaccount" -ExtendedRights Receive-As

* In the Exchange Management Shell, use the following command to
allow access to an individual mailbox:
Add-MailboxPermission -identity "user" -user "serviceaccount"
-AccessRights FullAccess

To do the same for Public Folders:

*To add AllExtendedRights permissions for the user Chris to access
the public folder named Marketing, run the following command:
Add-PublicFolderAdministrativePermission -Identity "\Marketing"
-User "Chris" -AccessRights AllExtendedRights -Inheritance SelfAndChildren