Outlook 2010 and newer
Outlook 2010 and 2013 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.
- Close Outlook application completely
- Type or paste %localappdata%\Microsoft\Outlook\RoamCache in the address bar of Windows Explorer then press Enter to open the folder location where the cache is stored in a file named Stream_Autocomplete_0_[long GUID].dat.
- Find the the largest file with name Stream_Autocomplete_0_[long GUID].dat
- Find the smallest one and copy the name including file extension (.dat)
- Delete the smallest file
- Create a copy of largest file and rename it to match the name of the smallest file you've deleted earlier
- Open Outlook and create new email message. Try typing a first letter in address bar and you should get a list pop up