AutoComplete feature in Outlook and how it works:
The nickname cache feature makes it quicker to resolve a name for a recipient that you use on a regular basis. The feature has changed a little over the years with the different versions of Outlook. For example, the filename extension has gone from .NICK to .NK2 between Outlook 2002 and Outlook 2003, Outlook 2007.
The NickName Cache file is used for easy resolving of names that you send emails most frequently. It saves you from having to type in the entire email address for a recipient.
The nickname cache file will be located on a user’s machine in the following path:
Windows Vista drive:\Users\user\AppData\Roaming\Microsoft\Outlook Windows XP or Windows Server 2003 drive:\Documents and Settings\user\Application Data\Microsoft\Outlook
Windows Vista drive:\Users\user\AppData\Roaming\Microsoft\Outlook
Windows XP or Windows Server 2003 drive:\Documents and Settings\user\Application Data\Microsoft\Outlook
When you use Outlook to send email it will store the email address for the recipient in the Outlook profile’s matching named NickName Cache File (i.e. Profile name: “Default Outlook Profile”, corresponding NickName Cache file: “Default Outlook Profile.NK2”). This is part of the Auto-Suggest/Auto-Complete component of Outlook.
If you have two computers and on the second one you have created a new profile, a new .nk2 file will be created. You can copy the .nk2 file from the first computer on the second one in order to maintain the cache names. Each profile will have a .nk2 file associated.
The nickname cache is always held in memory until you exit Outlook. The updated information related to the cache is not written into the .nk2 file until you exit Outlook. If you receive an NDR after sending to the recipient, the correct information for that user is no longer available in the NK2 file or the file got corrupted.
AutoComplete feature in OWA 2007 vs Outlook:
OWA allows now to cache the most recent entries used when sending messages. These entries are stored on the user’s mailbox. There is no link between these entries and the entries stored in the Outlook .NK2 file.
For example if you used Outlook to send messages to users: User1, User2, User3, in the .NK2 file of the Outlook profile you will have these entries.
If you use afterwards OWA 2007 to send message to User4, User5 in the OWA 2007 these entries will be available for the OWA AutoComplete. User4 and User5 will not be stored in Outlook’s .NK2
Option 1: Delete only the “incorrect” entries manually using Outlook
As this file is associated to each profile and stored on users computers, in order to remove only the corrupted entries, users should delete them manually from Outlook.
If there are few incorrect entries I would suggest this method because we will avoid losing the entire recipients cache. If the entire file is corrupted, you will need to recreated the entire .NK2 file.
To remove an user from Outlook's nickname cache (.NK2) file, clear the nickname cache entry for that specific recipient:
- Open a new e-mail message.
- Begin typing the first letters of that recipient's name or e-mail address.
- Continue typing the name until only that one recipient's name appears in the list of automatically suggested/completed names.
- Press the DEL key on your keyboard.
Note: One of the most important things to remember about the nickname cache feature is that new entries are not written to the .nk2 file until you exit and restart Outlook.
Delete a name from the AutoComplete list
Option 2: Delete the .NK2 file in order to have a new file created
In this case the entire file will be replaced with a new one. The existing entries in the old file will not be added in the new one. The new .NK2 file will be an empty file and entries will be created while messages will be sent to different recipients (after Outlook exit, these entries will be written in the .NK2 file).
Additional details on AutoComplete can be found here :
Using the automatic name fill-in feature (AutoComplete)
About AutoComplete name suggesting
Delete a name from the AutoComplete list