Delete Auto Complete Entries From Outlook
You may have noticed email addresses showing up when you start typing an email address to send mail to. This is Outlooks auto complete feature. You may want to remove old email addresses or miss typed email addresses from showing up.
Start a new message & click in the To… box
Start typing the email address you want remove & use the down arrow to select it.
Once it’s highlighted hit Delete & it will be removed.
You can delete all auto complete entries from outlook.
The file is here, where username is unique to your installation: “C:Documents and SettingsusernameApplication DataMicrosoftOutlook”
Delete or rename all *.nk2 files at this location.
Microsoft has a more detailed how to here: