Outlook Cached Address Book NK2 to RoamCache Migration: How to Force

When you upgrade Outlook from an older version that uses a .nk2 file for nickname autocomplete to a newer version that uses RoamCache, the migration of your cached address book entries may not happen automatically. This can leave you without your frequently used email addresses and contact suggestions. The migration is triggered by specific Outlook actions, but sometimes it fails silently. This article explains how to force Outlook to migrate your .nk2 data to the RoamCache store so your autocomplete list is restored.

Why Outlook Stops Using the NK2 File After Upgrade

Outlook 2010 and earlier versions stored nickname autocomplete data in a file named Outlook.NK2 located in %appdata%\Microsoft\Outlook. Starting with Outlook 2013, Microsoft replaced the .nk2 file with a new storage system called RoamCache. RoamCache stores autocomplete entries in .stream files under %localappdata%\Microsoft\Outlook\RoamCache. This change allowed autocomplete data to roam with your mailbox in Exchange cached mode and across multiple devices.

When you first start a newer version of Outlook after upgrading, the program looks for an existing .nk2 file. If found, Outlook attempts to import the entries into RoamCache automatically. However, this migration only happens once and only under specific conditions. If Outlook does not detect the .nk2 file at the right moment, or if the autocomplete feature is disabled, the migration is skipped. The old .nk2 file remains untouched, and your RoamCache folder is empty. The result is an empty autocomplete list when you compose new messages.

Steps to Force the NK2 to RoamCache Migration

1. Close Outlook completely
   Make sure no Outlook processes are running. Open Task Manager with Ctrl+Shift+Escape and check under the Processes tab for any outlook.exe entries. If present, right-click and select End task.
2. Locate your NK2 file
   Open File Explorer and paste this path into the address bar: %appdata%\Microsoft\Outlook. Look for a file named Outlook.NK2. If you have multiple profiles, the file may be named Outlook1.NK2 or use your email address as the filename. Note the full path and filename.
3. Back up the NK2 file
   Copy the Outlook.NK2 file to a safe location such as your desktop or Documents folder. This ensures you can restore the file if something goes wrong.
4. Clear the existing RoamCache folder
   In File Explorer, paste this path: %localappdata%\Microsoft\Outlook\RoamCache. Delete all files inside this folder. Do not delete the folder itself. This removes any partial or corrupted autocomplete data that may block the import.
5. Enable the autocomplete setting in Outlook
   Open Outlook. Go to File > Options > Mail. Scroll to the Send messages section. Check the box labeled “Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines.” Click OK to save.
6. Force import using the /importnk2 switch
   Close Outlook again. Press Windows+R to open the Run dialog. Type the following command, replacing the path with the actual location of your NK2 file: "C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE" /importnk2 "C:\Users\YourUsername\AppData\Roaming\Microsoft\Outlook\Outlook.NK2". Adjust the Office version path if needed (Office16 for Office 2016 and Microsoft 365, Office15 for Office 2013). Press Enter.
7. Verify the migration
   When Outlook opens, compose a new email message. Start typing a name in the To field that you know was in your old autocomplete list. Outlook should display suggestions immediately. To confirm the data is now in RoamCache, close Outlook and check the RoamCache folder again. You should see one or more .stream files with recent timestamps.

If the Migration Still Does Not Work

Outlook does not have an NK2 file to import

If you never used Outlook 2010 or earlier on this computer, or if the NK2 file was deleted, there is no data to migrate. You must rebuild your autocomplete list manually by sending emails to your frequent contacts. To speed this up, you can export contacts from your address book and send a test message to each one.

The /importnk2 switch is not recognized

This switch works only in Outlook 2013, 2016, 2019, and Microsoft 365. If you are using Outlook for Mac or the new Outlook for Windows (the preview version), the NK2 migration is not supported. You must use a third-party tool or manually re-enter contacts.

RoamCache files are created but autocomplete remains empty

This usually means the autocomplete setting was disabled when the import ran. Go to File > Options > Mail and confirm the “Use Auto-Complete List” box is checked. If it is already checked, uncheck it, restart Outlook, then re-check it and restart again. This forces Outlook to reload the RoamCache data.

NK2 File vs RoamCache: Key Differences

Item	NK2 File	RoamCache
Storage location	%appdata%\Microsoft\Outlook	%localappdata%\Microsoft\Outlook\RoamCache
File format	Single .nk2 binary file	Multiple .stream files per mailbox
Outlook versions supported	Outlook 2003 through 2010	Outlook 2013 and later
Roaming support	Local only, does not roam with Exchange	Roams with Exchange cached mode and Microsoft 365
Import trigger	Manual copy or upgrade	Automatic on first startup after upgrade
Recovery method	Replace file while Outlook is closed	Delete .stream files and force re-import

Now you can force Outlook to migrate your old NK2 autocomplete entries into the modern RoamCache system. After the migration, your frequently used email addresses appear instantly when composing messages. To keep your autocomplete list in sync across devices, ensure your Outlook profile is configured for Exchange cached mode. As an advanced step, you can export the RoamCache .stream files as a backup by copying the entire RoamCache folder while Outlook is closed.