In Tools | Options | Email Options | Advanced E-Mail
Options, there is a setting: "Suggest names while completing the To, Cc and Bcc fields."
When enabled, Outlook suggests matches as you're typing in these fields, much like
IE will suggest URLs as you're typing in the Address bar. Type in "David" and you
will get a list of Davids to whom you have sent mail in the past (names are added
to this cache on resolve, i.e. once they're underlined).
After a while, this list could get cluttered with people
you rarely send mail to, so you don't need them in the cache. So you can delete those
entries from the cache:
1. Type enough characters so that the name shows up
in the list
2. Using the down arrow, select the entry you want to delete
3. Press the delete key
Buh-bye goes the name.
Note: The cache is stored on the client, so if you
move to another computer with Outlook on it, you won't have the same list. For information
on how to back-up this cache as well as the rest of your Outlook data, see http://support.microsoft.com/?id=287070.
For information on how to configure the number of names that show up in the cache,