The autocomplete cache:

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Potential Addressing Problems for Outlook Users:

Certain changes on the Exchange side can/have invalidated cached entries on our Outlook clients. This results in mis-addressed e-mails bouncing and confused end users. The solution is to delete the autocomplete cache file---which leads us to another issue. An inherent problem with the autocomplete cache is its seeming fragility. Users have difficulties in adding addresses to their Contact folder, so they rely heavily on the autocomplete cache. Then the cache gets hosed and...

Here's How the Cache Works:

"We use Outlook 2003 and my users aren't 
very good at adding email addresses to the 
Contacts folder and rely heavily on the 
autocomplete cache, but it gets replaced 
often. Why does it seem so fragile?"

First, I'll explain how the autocomplete 
cache works. The autocomplete cache is also 
known as the nickname cache and is stored as 
a hidden file in C:\Documents and Settings\
username\Application Data\Microsoft\Outlook\
profile_name.NK2 if you use Outlook 
2002/2003. It has a limit of 1000 addresses, 
and new addresses are not added to it until 
you end the current Outlook session. There 
is a registry key you can edit to control 
the size of the nickname cache in older 
versions of Outlook, but it doesn't work 
with Outlook 2003. 

If the users are losing addresses of people 
they recently replied to, it's because 
Outlook crashes or does not close properly 
and the changes are not written to the NK2 
file. The existing *.NK2 shouldn't be replaced 
by a clean *.NK2 if this happens, but new 
addresses you've sent new mail to or replied 
to won't be saved to the file. Less used 
addresses are replaced by new addresses when 
the NK2 contains 1000 addresses, but if 
something is corrupt, Outlook replaces the 
current NK2 with a new, clean one. 

If you have a mistyped address in your cache 
you can use the arrow keys to select the 
address and press Delete to remove it. You 
can also delete the existing NK2 file to 
completely clear the cache and start with 
a clean cache. 

Microsoft doesn't provide a tool to edit the 
nickname tool, however, if you need to view 
or edit the NK2 file, a third party utility 
called Owtlook is available. This tool allows 
you to edit the NK2, capture the addresses and 
create contacts, or populate it with 
frequently used addresses. Owtlook is 
especially useful for corporations that need 
to capture all addresses employees correspond 
with. 

If you're using the autocomplete cache as your 
address book because it's too much effort to 
right click on an address and save it as a new 
Contact, you should use a utility that creates 
contacts from all addresses you reply to, as 
the autocomplete cache is not a good substitute 
for contacts.

You can find a list of utilities at http://www.slipstick.com/addins/contacts.htm#data

Note: If you use an older version of Outlook, much of the above information applies, but the nickname file for older versions uses the extension .nick.

Owtlook
http://www.dcs-imaging.com/products.htm

Reduce the Number of Names in the Nickname Cache (Outlook 2002)
http://support.microsoft.com/?kbid=293032

Other vaguely related points of interest regarding autocomplete entries

For mailboxes, one will see a "legacy dn" entry within an autocomplete item such as: "<legacy dn>". This is tattooed on the object and will list the original Gatorlink username even if someone has later changed it. May bother such folks, but it cannot be helped.

For locally stored contacts, the email address will be appended, similarly within "<>" brackets such as: "<e-mail address>". This is simply an oddity of how the autocomplete items display within the Outlook user interface.