On behalf of messages

My director would like for his assistant to send emails labeled from the assistant on his behalf. I set the permissions under the Delegate tab to Editor and also on his Outlook folder. When she sends a new message, adds his name to the from box, the message shows only from the director, not on behalf of. I set the same permissions for myself in his Outlook, and am able to send messages on his behalf. Our permissions are identical, yet messages from the assistant show only from the director. I have checked Outlook versions (2003) on all computers. We use Exchange 2003. Any ideas would be greatly appreciated.

Thanks