Here's how to stop Outlook from automatically filling in the e-mail address when typing the name of a contact in the To: field.
Outlook's Suggest Names feature helps speed up routine e-mail activities. However you may want to get rid of it when, for instance, you have two or more e-mail addresses that are similar and Outlook keeps choosing the wrong one.
To turn this feature off:
Select Options from Outlook's Tools menu
Click on the Preferences tab
Clickthe E-mail Options button
Click the Advanced E-mailOptions button
Uncheck the box, near the bottom of the dialog, labeled: "Suggest names while completing To, Cc, and Bcc fields"
From Slipstick.com: Suggest names doesn't work? If Auto Suggest and name checking is not working, rename the *.NK2 file. Outlook will create a new one. You can rename or delete the NK2 file if you want to get rid of all addresses in your autocomplete file.