NK2 file is by far the smartest option provided for the convenience of Outlook users. Once contact is made with an email address from your end, the NK2 file automatically stores it, whether or not the contact has been added to address book. Later on while typing the same email address in any of the contact fields (To, Cc, or Bcc) the NK2 which is better known as the Auto-complete file will portray its role.
It will auto complete the halfway written email address for your convenience and save you from having to remember or type in the entire address on your own. But completely relying upon the autocomplete file can be risky. Not adding the addresses to contact list and just depending on autocomplete could them vulnerable to loss. vCard files are easier to manage as compared to autocomplete files as not many are aware of the latter. Thus, the vulnerability of autocomplete files can be encountered while switching to a different Outlook version or to a different machine. Nevertheless, the following section will be discussing a workaround, applying which will help import contacts from an NK2 file to both; a different machine and version of Outlook.