The NK2 file stores all the AutoComplete information used by Outlook to “auto complete” the e-mail addresses you use. I discovered it a few weeks ago while getting ready to launch the QuietSpacing(tm) blog. The immediate value of the NK2 file is that it contains all the e-mail addresses of people with whom you’ve communicated – even those that aren’t in your Contacts yet!
There are a number of free utilities you can download that will allow you to access and manipulate the Nk2 file data, including saving it to spreadsheets and uploading it into other programs – like an e-mail newsletter list. Here’s the one I used:
but a Google search on “NK2 file” will turn up a host of other sites with information and utilities about this little buried treasure!