Outlook - user does not have permission to send on behalf

[Ducatista]

I have a user that I want to add as a delegate as send-on-behalf of, but when I go into options -> Delegates I get the following error message:
"The Delegates settings were not saved correctly. Unable to activate send-on-behalf-of list. You do not have sufficient permission to perform this operation on this object."

In the active directory, I gave the user that I want to be a delegate the permissions in MailBox Rights.

Am I missing permissions somewhere else?

Thanks,
Ryan