If you're having trouble getting Microsoft Outlook remembering to whom you've sent emails and auto-completing the To field for future emails, give this a shot:
- Open Microsoft Outlook
- Click "File"
- Select "Options" from the left hand menu
- Select "Mail" from the left hand menu
- Scroll to the "Send messages" section
- Click the "Empty Auto-Complete List" button
- Close Outlook and re-open
Email addresses should now auto-complete after the first time you send to that email address.
BONUS TIP: If you're sending an email to someone in your company address book, you can use Outlook to help you complete the name/email address. Example: you're sending an email to your co-worker Clint for the first time. Type "Clint" into the To field of the email and press Ctrl+k. If there is only one Clint in your address book, Outlook will automatically select him; if there's more than one it will popup a list of Clints from which you may select.