Unfortunately, Outlook remembers the mistyped and old as well as the correct and current and suggest it indiscriminately. But, getting rid of entries you no longer want to appear in the auto-complete list is easy.
You can also stop Outlook from suggesting email addresses as you type:
* Select “Tools” under menu and then select “Options” from the menu.
* Go to the Preferences tab.
* Click E-mail Options
* Now click Advanced E-mail Options
* Make sure “Suggest names while completing To, Cc, and Bcc” field is not checked.
* Click OK.