So I have a Mailbox that I needed a security group Send As permission as well as full access to the mailbox.
I am currently running Exchange 2003 with a Coex Exchange 2010 (In the process of migrating).
When I grant the Security group or the individual user Send As rights the get a message stating they do not have permission to send on behalf of,
Not sure why this is the case. I am able to send as user using my account (Domain Admin).
If I do grant Delegate Rights to send on behalf of the users are able to send messages from the mailbox (within the profile) as if it is coming from the mailbox. If I mount the mailbox to the current user Outlook Profile the message states that the message is sent on behalf of.