Outlook 2007 improves user productivity by caching the most recently used e-mail addresses entered in the TO:, CC:, and BCC: fields when messages are sent. This cache is stored in an AutoComplete file with the extension .NK2 located in the user’s profile, for example in Windows Vista it would be:
As the user types an address, the cache items matching the letters typed are displayed in a dropdown box [See Figure 1]. The user can navigate through the cached items with the arrow keys and select the desired recipient if it is present. By the same manner, the user can delete individual entries in the cache as well. The AutoComplete file handles up 1000 entries and stores more than just addresses. Among its content resides X.500 directory information, X.509 PKI certificate data, and auto-resolution information as well.
If there are many addresses in the NK2 file that are no longer desired or the file is not providing information correctly through possible corruption, the file can be deleted. Outlook will create a new one with no entries next time that profile is opened as outlined in Microsoft KB article 287623. The cache will build again as the user sends mail.
Not everyone may want this feature. The added convenience may increase the potential to send confidential content to an unintended recipient. AutoComplete is easily toggled on or off through Outlook. Navigate to Tools | Options | E-mail Options | Advanced E-mail Options. To disable AutoComplete, clear the checkbox beside the option to Automatic Name Checking and Suggest names while completing To, Cc and Bcc fields options [See Figure 2]. The former checks address books and the cache to resolve recipients while the latter is responsible for displaying the dropdown box for AutoComplete.
While intended as a productivity feature of Outlook, some have grown to depend on the AutoComplete cache almost as an address book. There are third party applications that can parse the .NK2 file and present its contents in an interface through which users can administer their AutoComplete file. Ingressor and NK2View are two examples of Outlook AutoComplete management tools.
For more tips, visit a collection of popular Outlook help articles on the Outlook tips page.