You can clear the entire autocomplete file: in older versions you need to delete the *.NK2 file from your hard drive, while Outlook 2010 and newer has a button to clear the Autocomplete cache in Options (screenshot below).īeginning with Outlook 2010, Outlook does not use the NK2 file it stores the autocomplete cache in the mailbox or data file and caches the addresses in an autocomplete stream at C:\Users\%username%\AppData\Local\Microsoft\Outlook\RoamCache.