Do you currently use Outlook? If so, you might be in a problem like a lot of my clients or other users are finding out. Too many people are so used to the Web Mail which will automatically add your email contacts into your address book. If you use Outlook, and used 2000 or prior, it would be set that it will automatically put each email address in your address book.
Too many people rely on the Auto Fill to remember the email as a quick shortcut. Keep in mind, you can do this, BUT this doesn’t put the person in your address book.
You can always Right click on the email and Left Click on Add to Address Book. Same when you look at the emails you can Right click on the person’s name too.
Another reason why you want to add them to your address book if you do a backup of Outlook to a PST file (This will copy emails, tasks, addresses, etc). If you haven’t done this or not sure what this is, you can check out the blog I did which tells you step by step how to do a backup.