Hello Richard,
Your question seems to be specific to an on-premise Exchange environment, and not to our services. These are the forums for the Business Productivity Online Services and therefore I am unable to help you with this problem.
As a general remark however, send-on-behalf permissions does not require setup by the Exchange Admin on the server level, but rather can be set in the Outlook client by the user using delegation.
If this feature does not function as expected, you should consider posting your question on this forum http://social.technet.microsoft.com/Forums/en/category/exchangeserver
Hope this helps,
Justin Bakelaar – BPOS Support