Every time you send a new email message in the Outlook client, the addresses you type in the 'Para' and 'Cc' fields are automatically inserted in the autocomplete list, so that next time the program memorizes them and you don't have to rewrite them
The Outlook autocomplete file is stored in [Your Profile] Application DataMicrosoftOutlook, and is always saved with the profile name and the.NK2 extension. This program, called NK2View, is used to read the autocomplete.NK2 file, shows all the addresses stored in it and allows you to export this information to text / html / xml format files.
You can free download NK2View 32, 64 bit and safe install the latest trial or new full version for Windows 10 from the official site.