Found this useful article that explains Outlook's Auto-Complete feature:

http://www.outlookpower.com/issues/issue200506/00001584001.html

"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 aren't 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 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. We don't know what causes it to become corrupt, but we suspect a large number of addresses in the cache is part of the problem."