We are using exchange 2010 SP1 in our environment. I have one user no matter who you try to setup so she can send on behalf it always comes back and says you do not have permission to do it.
We set the person up with full access to the mailbox, with send as on mailbox that did not work. Next we set the person up with send on behalf using the mailbox properties still same error. i went on the AD account and went to permissions and added that person to the AD account with send as and full control and still nothing is working. Any other users you allow send on behalf for any mailbox is working just fine.