Solution 1: Manually Delete (For ALL Versions of Outlook)
1. Open Outlook. 2. Type the e-mail address you wish to clear from the cache. 3. Press the Down-Arrow button (on the keyboard) to select the e-mail address. 4. Press the Delete button (on the keyboard). 5. That e-mail entry is now removed from the Outlook Auto-complete cache.
Solution 2: Empty Auto-Complete List Button (ONLY for Outlook Versions 2010 & 2013)
1. Open Outlook. 2. Click File | Options. 3. Click on the Mail tab on the right. 4. Scroll down to Send messages and click the Empty Auto-Complete List button.
Solution 3: Recreate the .nk2 File (ONLY for Outlook Versions 2003 & 2007)
1. Close Outlook. 2. Open Windows Explorer or Internet Explorer. 3. Paste the following into the Address Bar: %APPDATA%\Microsoft\Outlook 4. Delete the following file from this folder: Outlook.nk2
Important Note: This will delete ALL of your cached e-mail addresses. It should ONLY be used if you want to wipe clean your cache or if there is corruption issues in your .nk2 file. The first time you open Outlook after deleting this file, Outlook will create a NEW .nk2 cache file automatically & start caching the e-mail addresses you use from here on out.