Clear the Auto-Complete Email Address Cache in Outlook

From mail2web.com Wiki

Outlook 2003, Outlook 2007 and Outlook 2010 have an automatic completion feature for any email address that you try to type into the TO or CC fields. This feature does not pull from the contacts or address book. If you have emailed somebody that wasn't in your contact list it will still add that to the auto-complete list.

The problem occurs when you use a bad/wrong email address, and then every time you try to type in the email address the incorrect one appears in the auto complete list. To fix this problem, there are two ways around this. The easiest way is:

• Start typing in the address that shows up incorrectly.
• Press the Down Arrow key to select the auto completed address.
• Press the Delete key to delete the auto complete address.

The other way is by opening your Outlook profile folder and deleting the .NK2 file which holds the entire auto-complete email cache.

• From the Start Menu select Run...
• Type in the following:

%APPDATA%\Microsoft\Outlook

• Delete the Outlook.NK2 file that you find in this folder.

You can also clear the AutoComplete cache by doing the following:

• CLick Start and then Run
• Type in the following:

Outlook.exe /CleanAutoCompleteCache

Source: How to clear or delete the Autocomplete Cache/List in Outlook 2010