Outlook keeps almost all of your data in a special personal folder file (.pst). That includes the AutoComplete list. That’s the list that pops up when you start typing someone’s name or address in a new message. This list is a special tool that Outlook stores in a separate file. It’s easy to take for granted — until it stops working correctly, or isn’t available at all.
Making a backup is easy; finding the file is the hard part. To find the Outlook file that contains your AutoComplete list, do the following:

  1. Close Outlook.
  2. Open Windows Explorer and enter the following address in the Address control: %appdata%\Microsoft\Outlook. Windows Explorer will update to the appropriate Outlook folder.
  3. Right-click the Outlook.nk2 file. It may have another name, but there should be only one file with the .nk2 extension.
  4. Select Copy from the resulting submenu. From there, you can copy the file to an external drive, USB drive, or CD — wherever you store your backup copies. If you copy the file to another folder on the same local drive, be sure to rename it to avoid problems.

If you have to restore the file, use the same method to find the right Outlook folder and then write over the existing .nk2 file, if it’s still there.