This isn’t a problem I’ve ever experienced but I had to help a colleague out with it today, and it seems to be relatively common.
The problem is where Outlook’s AutoComplete feature for remembering previously used email addresses just doesn’t work, each time you open Outlook-they’re all gone which is pretty annoying. It all comes down to the “.NK2” file that Outlook 2003/2007 uses and there are 2 likely causes:
1) Your .NK2 file has become corrupted somehow
2) You upgraded from Outlook 2000 and the new file wasn’t created.
Outlook 2000 used a “.NICK” file which is unreadable to later versions of the email client, but it seems that an upgrade from 2000 to 2007 doesn’t replace it with the required “.NK2” file. This was the case at work so we simply closed Outlook/deleted the “.NICK” file (some prefer to rename the file rather than deleting it)/re-opened Outlook/sent an email/hey presto a new “.NK2” file appeared and all was well 🙂
The file can be found here:
C:\Documents and Settings\<your username>\Application Data\Microsoft\Outlook
I hope this helps!
Cloudy with a chance of Licensing 
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ebaresC.
Thanks for the post. Did the trick.
I’ve been trying to fix this problem for a while now.
Thank you so much. I have been trying to resolve this all day. I have followed many other instructions on the web to no avail. Then just before going to bed I thought … just once more. Eureka! Thank you, and goodnight.