Microsoft Exchange Server 2007 does not support to assign a mailbox with the "Send on behalf" permission of a security group in the Exchange Management Console (EMC) or in the Exchange Management Shell (EMS).
Note You can assign this permission to a mailbox by adding the security group as a delegate in Microsoft Office Outlook.
To resolve this issue, install the following update rollup:
2407025 Description of Update Rollup 2 for Exchange Server 2007 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about how to create a security group, visit the following Microsoft website: