Outlook provides a cache of email addresses as you use them in new email messages. This cache, sometimes called a nickname cache, is intended to improve user productivity. As you type an address in an email address field, Outlook lists possible addresses matching the letters you’ve entered.

Related: Import Outlook autocomplete (NK2) files into Microsoft Outlook 2010

Figure 1 shows an example of this feature.

Figure 1: An example of suggested email addresses in Outlook as you type
Figure 1: An example of suggested email addresses in Outlook as you type

Historically, Outlook stores this cache in a local, profile-specific file with the extension .nk2. Also, Stephen Griffin of Microsoft, who also maintains MFCMAPI, posted a document outlining the interaction between Outlook 2003 and 2007 with the .nk2 file; see "The Nickname Cache ." 

Outlook 2010 changes the location of the AutoComplete Cache. It’s no longer an .nk2 file but is now a hidden folder within an Exchange Mailbox or a .pst file, depending on the account type in Outlook. The principle is the same, of course; however, with the nickname cache in the mailbox, the content now travels with the user. When you recreate a user’s profile on another workstation, their nickname cache is unaffected. With Outlook 2003 or 2007, the nickname cache would be recreated as the user began sending new email messages.

Related: Clearing the Outlook Auto-Complete Address Cache

Outlook 2010 also adds a folder to Contacts labeled Suggested Contacts. Addresses get added to this folder as you send or reply to messages addressed to recipients who don’t already exist in your address book (aka your Contacts folder). That sounds very simliar to the AutoComplete process! AutoComplete is a productivity enhancement that shows an address as you start to type a common recipient address so you can select the address instead of typing it. Suggested Contacts are addresses that you might consider adding to your Contacts, and Outlook makes it easier to do so. You can double-click an entry in Suggested Contacts and a Contact Form opens that allows you to fill in more information and then save it to your Contacts Folder. Figure 2 shows the Suggested Contact folder with a few recently used recipients.

Figure 2: The Suggested Contacts folder
Figure 2: The Suggested Contacts folder

Over the years, Microsoft has fielded numerous requests for formal, supported access to the AutoComplete cache. Third-party companies even produced rudimentary .nk2 file editors. Suggested Contacts is a way for administrators and users to control AutoComplete cache contents directly through the Outlook interface. Addresses added to Suggested Contacts are used for AutoComplete and addresses entered as recipients of email messages that do not reside in the user’s Contacts folder are added to Suggested Contacts. You can copy and paste contacts from a contacts folder to Suggested Contacts to add their addresses to your AutoComplete cache.

Administrators or users could pre-populate the Suggested Contacts folder with commonly used company addresses to improve messaging productivity. AutoComplete has evolved from a productivity enhancement to a functional feature. In a future Outlook Tips and Techniques column, I’ll look at how to turn off AutoComplete functionality and give you some reasons why you might want to do so.

Related: The Implications of Outlook 2013 Changing OST Cache Behavior