I doubt you want to turn off "automatic look up" (automatic name checking).
You want to turn off autocompletion.
Feature already exists.
You can turn off either or both autocompletion (Suggest Names) and
autoresolution (Automatic Name checking) in Tools > Options... > Perferences
Tab > Email... > Email Options... > Advanced Email options...

Signature
Russ Valentine
[MVP-Outlook]
Many thanks. But I want to turn this off for SELECTED contacts only!
> I doubt you want to turn off "automatic look up" (automatic name checking).
> You want to turn off autocompletion.
[quoted text clipped - 17 lines]
> >
> > http://www.microsoft.com/office/community/en-us/default.mspx?mid=a72de18b-a840-4
1d5-863e-e988b7f6b679&dg=microsoft.public.outlook.contacts
Russ Valentine [MVP-Outlook] - 30 Nov 2005 15:10 GMT
That would be an infrequent request indeed, I suspect.

Signature
Russ Valentine
[MVP-Outlook]
> Many thanks. But I want to turn this off for SELECTED contacts only!
>
[quoted text clipped - 23 lines]
>> >
>> > http://www.microsoft.com/office/community/en-us/default.mspx?mid=a72de18b-a840-4
1d5-863e-e988b7f6b679&dg=microsoft.public.outlook.contacts
Brian Tillman - 30 Nov 2005 19:08 GMT
> Many thanks. But I want to turn this off for SELECTED contacts only!
The problem with that thinking is twofold. The first is that autocompletion
has nothing to do with your contacts. The second is that the autocompletion
cache can't predict what address you're _thinking_ about using before you
start typing it.

Signature
Brian Tillman
mehrdad - 01 Dec 2005 04:48 GMT
AS Russ correctly points out that would be infrequent. Although I have
response to the problem posed by you, I as a user am not concerned with the
mechanics of the matter, I have a practical need to be addressed: I want
'look-up' feature to be on and I want this to be able to turned it off (and
later turned on) for the contacts I select. This command should be available
on the right click short-cut menu on the contact. This way I would not
mistakenly send a message to someone to whom I have only sent a message or to
sometime ago instead of friend to whom I sent a mail every other day.
> > Many thanks. But I want to turn this off for SELECTED contacts only!
>
> The problem with that thinking is twofold. The first is that autocompletion
> has nothing to do with your contacts. The second is that the autocompletion
> cache can't predict what address you're _thinking_ about using before you
> start typing it.