I possibly know the outcome to this problem, but I've been asked to investigate anyway.
I work in a law firm that has a lot of very sensitive information. Occasionally, an under-pressure-super-busy secretary may send an email to who they thought was another staffer in the office, but was in fact a client for an unrelated matter (eg. same first names).
A director has asked me, and said that he himself has done the same as the staffer before, to ensure that only authorised or internal email addresses are suggested when you compose an email.
I explained that the auto-complete feature is a blanket on or off setting that is there to assist you by suggesting previously contacted addresses with the same characters you've typed. But he comes back at me with his authoritative demeanor that it is an expectation in a corporate environment that such a feature be restricted to internal email addresses only ("What happens in the Prime Minister's office for their staff?") and to find a solution for our office.
I can empathise with him and understand why he believes that feature should exist, so here I am, ticking the boxes.
Is there any way of restricting the auto-complete list in Outlook?
The best I can think of is turning auto-complete off, clearing the staff's NK2/ACL and using Nk2edit to import only the Exchange GAL. But that won't work and it won't suggest any of the GAL addresses because auto-complete is off. Turn it back on and it starts learning again. So leave it off and use auto-resolution and/or the actual address book (who knew such a thing existed?!)?