Consider the following scenario:
You are assigned the "Full Access" and "Send on Behalf" permissions on a secondary mailbox in a Microsoft Exchange Server 2010 environment.
You use Microsoft Outlook or Outlook Web App to log on to the secondary mailbox by using your own credential.
You send an email message from the secondary mailbox.
In this scenario, the email message is sent by using the "Send As" instead of "Send on Behalf" permission.
To resolve this issue, install Update Rollup 8 for Exchange Server 2010 Service Pack 3.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.