Hi, Are you still having this issue?
If so there are a few things you can check, first of all, what URL is the link taking the user to, is it Service Manager (perhaps you could provide the link for some insight)? If so the user might need additional capabilities to access Service Manager.
Secondly, at what point in the delivery phase is this mail being sent, is this upon completion of the service request or during service delivery? If it is during service delivery then you would have to configure access control during the service delivery moment and not during service completion for the user to access it.