On Outlook, the Automatic Completion and Name checking do not work properly or How to Reset the Nickname and Automatic Completion Cache
Symptoms
Outlook maintains a nickname list that is used by both the automatic
name checking and the automatic completion features. The nickname
list is automatically generated as you use Outlook. If the nickname
cache is corrupted, Outlook may not be able to identify recipients,
may offer incorrect recipients when automatically completing the
e-mail address, or may send the message to the wrong person.
Solution
Use the following steps to reset the Outlook nickname cache.
Quit Outlook.
Open C:\Documents and Settings\<username>\Application Data\Microsoft\Outlook.
Search for File with extension *.NK2.
Right-click the .NK2 file with the name of the profile that you want to reset, and then click Rename.
Rename the file to <profile_name.bak>
Restart Outlook.
Outlook will generate a new nickname cache.
Note: If you try to locate a NK2 file with Start | Find
on a Windows 2000 or XP system, you need to be aware that
Windows treats these files as hidden files. In Windows Explorer, use
Tools | Folder Options | View to make sure that hidden files
and folders are shown. By default the folder "Application Data"
is hidden by Windows.