Blog @ E-knows.net

There are two methods to clear email address cache from the outlook.

1) As you type the email address, use drop down key to select the email address. Press delete to delete (Not backspace)

2.1) Close Outlook
2.2) Open Windows Explorer or IE and Paste %APPDATA%MicrosoftOutlook into the address
bar.
2.3) Delete the file Outlook.nk2 file in this folder
2.4) All Done.