It gets the addresses from the Contacts folder. Make sure your address book entries are also in the Contacts folder. Open the Address Book, Tools, Options and check the settings there. There are 3 or more address books that can be the same or different. Outlook uses the Contacts--Outlook Address Book in my setup. If you don't use Outlook for business email & contacts, it's much easier to use Outlook Express.

Thanks for your help.  All my addresses are under Contact.  I found out that Outlook only autosuggests To: names that have been entered since the program installation.  The suggestions are stored in a .nk2 file in Documents and Setting/application data/microsoft/outlook.  Does anyone know how to fill a .nk2 file with all the contact names to shorten the learning process?  Thanks.

That's not true. It will suggest names from the Address book even those from before installation. Right click on Contacts folder, choose Properties, Outlook Address Book, check Use this folder as an Address Book. To quickly add to your list, right click on any email address in the To and From fields and choose Add to Contacts.

Right clicking on a new To: name will add it to the Contacts address book, but unless the name was previously used in a sent email, the full name of the new contact will not automatically fill until the "TO:" button is clicked.  For example, say I receieved a email from Dell Administrator.  I can right click as you described on the From: field in the email and directly add it to my Contacts Address Book.  However, the first time I send an email to Dell Administrator, as I type "De" it won't automatically offer "Dell Administrator" as a choice for completion.  The next time I send an email to Dell Administrator, all I have to do is type "D" and it offers "Dell Administrator" as an autocompletion choice.  A minor issue, but a convenience lost when I imported all my contacts from one computer into my new notebook.

It adds the address, but you have to fill in the rest and check it for accuracy. Do you have Auto complete checked in Outlook?

Yes.  The "suggest names while completing To:, etc" under advance email options is checked.  My understanding is that the .nk2 file remembers the completed name for each sent email, and then uses that list of names as a basis for automatically completing future emails.  Its a learning process that for most people evolves as contacts are emailed.  For importing a new contact list, it must be relearned to build up the .nk2 file.  Conversly, if someone is pestered by lots of old outdated completion suggestions (say addresses no longer valid), they can erase the .nk2 file and Outlook will start another blank one from scratch, but this erases all the suggestions.  Thanks for your help.