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If you are setting up Outlook on a new PC for a user and they have a need to be able to retain the autocomplete when typing in an e-mail address...

Outlook 2003/2007 the file you will need is .nk2
Earlier than Outlook 2003 you will need to find the .nck or .nick file

How to backup the Cached Email addresses from Outlook
The file(s) you are going to need is located in the users profile directory;
•Windows 7 - C:\Users\<user>\AppData\Roaming\Microsoft\Outlook
•Windows Vista – C:\Users\<user>\Application Data\microsoft\Outlook
•Windows XP or earlier – C:\Documents & Settings\<user>\Application Data\microsoft\Outlook

Copy the file(s) and back it up to another location like a flash drive or network share.

How to restore the cached email addresses from Outlook.
Basically all you have to do is reverse the backup process.
1. Make sure that you have created the profile for the person both on the computer and in Outlook and it has been logged onto.
2. Exit out of Outlook
3. Copy the file from the flash drive/network share back to the appropriate user profile directory for Outlook.
4. Run Outlook again, create a new e-mail and begin to type an e-mail address you know they have previously used on the other PC

You should see it complete the address as on the other PC...

There is a program called “NCKCSV”. This program allows you to backup your “quick outlook contacts” and save them as a CSV file. Once you have done that you can import them into your contacts in outlook. I have not personally used this, but I have heard it works fine.

Extra to this.

If you are setting up Outlook on a replacement PC for a user and copy their .nk2 file to the new PC and it is running Outlook 2010 or later, the cached addresses may not show up. If that is the case try this...

1. Make sure that the .nk2 file is in the following folder: %appdata%\Microsoft\Outlook
Note The .nk2 file must have the same name as your current Outlook 2010 profile. By default, the profile name is "Outlook." To check the profile name, follow these steps:
•Click Start, then click Control Panel.
•Double-click Mail.
• In the Mail Setup dialog box, click Show Profiles.
• Rename the file if needed

2.Click Start, and if running XP click "Run" if you are using Winodws Vista/7 then just type the bolded part of the sentence below into the search box after having pressed the Start button

3.In the Open box, type outlook.exe /importnk2, and then click OK. This should import the .nk2 file into the Outlook 2010 profile.

After you import the .nk2 file, the contents of the file are merged into the existing nickname cache that is currently stored in your mailbox.
The .nk2 file is renamed with a .old file name extension on the first start of Outlook 2010. Therefore, if you try to re-import the .nk2 file, remove the .old file name extension.

This is if you need to transfer Outlook 2010 cached addresses to another PC running Outlook 2010

1. Browse to
C:\Users\%username%\AppData\Local\Microsoft\Outlook\RoamCache

The Auto Complete cache is stored in a file that begins with the text “Stream_Autocomplete_” and has a file extension of .dat

If you have multiple Outlook 2010 profiles on your Windows login you will almost certainly see multiple Stream_Autocomplete files as each profile creates its own cache file. A Stream file will only be created when you have sent an email in the new profile.

You should be able to determine the file you need by either looking at the date modified or the file size.

2. To copy the autocomplete information from one machine to another we have to tell a new Outlook profile to use the old autocomplete file. We do this by:

a. Log the user on to the new machine, configure Outlook and send a test email. Then close Outlook.
b. Determine on the old computer which Autocomplete file holds the autocomplete information you need using the above, then make a copy.
c. Copy the file to the same location on the new computer.
d. Find the new Stream_AutoComplete file (the one with the smallest file size and newest date) being used by your new Outlook profile, right click it and select rename, then hold down ctrl+c to copy the filename (not the file) to the clipboard. Finally, rename this file with a .old extension.
e. Now, go back to the copy of the file you made earlier, choose to rename it and press ctrl+v to paste the new filename.
f. Re-open Outlook on the new computer and test.

If all is well, you should see the autocomplete working as it did on the old PC...