Found the answer...
http://www.outlook-tips.net/archives/2004/20040223.htm
What is Autoresolve and Autocomplete?
All versions of Outlook support Autoresolve. Also known as automatic name
checking, this feature allows you to type a name, or even just the first few
letters, and the name resolves a few seconds after you tab out of the field.
When one match is found, the name has a solid underline. When more than one
possible match exist you'll see a red wavy underline. Right click on it to
choose an address. When more than one match exists for a name you've used
before, Outlook resolves to the last name used and uses a green dotted line
to alert you that other possibilities exist. Right click to see the other
addresses. When Name checking is disabled in Tools, Options, Email Options,
Advanced Options it won't do it as you type the names in but it will check
and resolve them when you hit Send. You can force an autoresolve at anytime
using Ctrl+K.
Only Outlook 2002 and 2003 support Autocomplete. Also known as suggest
names, this is the feature that shows addresses of people you've sent
messages to previously and you can select matches as you begin typing. You
can't seed the list with names from your address book, the feature collects
them when you send new messages or reply. You can clear the autocomplete
cache by deleting the *.NK2 file for your profile or delete one address at a
time by using the arrow key to select it as you type and then Delete. Like
autoresolve, you can turn it on or off in Tools, Options, Email Options,
Advanced Options.
