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. (Source: WindowsITPro)
Historically, Outlook stores this cache in a local, profile-specific file with the extension .nk2. 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.