Outlook automatically completes frequently used email addresses or aliases; that is, addresses or aliases you enter repeatedly are cached and appear automatically after you type the first few letters.

Occasionally you need to delete one of the autocomplete addresses (e.g., when it is a misspelling or an alias changes). You can either delete individual cached autocomplete addresses or empty the entire cache. Follow the appropriate steps below.

Delete an individual address from the autocompletion cache

1. In Outlook, start typing the address as usual until the list of autocomplete addresses comes up. Often the list contains only one address, but there may be many more.
2. Use the arrow keys on the keyboard to select the address you want to delete. If the list has only one item, that item should already be highlighted.
3. Press the Delete or Del key.

Clear the entire autocomplete cache

Occasionally, especially in the case of misspellings, you have to clear the entire autocomplete cache. Occasionally that cache is corrupted, and attempting to delete an individual entry fails. In other cases, you may want to delete so many entries that it's easier to clear the whole autocomplete cache. To clear the cache, follow these steps:

1. Close Outlook.
2. Configure Windows to show hidden files:
   • From the desktop or the Start menu, double-click My Computer.
   • From the Tools menu, select Folder Options.
   • Click the View tab and select Show hidden files and folders (Windows 2000 or XP).
   • Click OK.
3. Search for files on your hard drive that contain the keyword *.nk2:
   • From the Start menu, select either Search or Find, and then Files or Folders... or For files or folders... . A dialog box titled Search Results will appear.
   • In the resulting dialog box, in Windows XP, click All files and folders. Then, in the "All or part of the file name" field (Windows XP), the "Search for files or folders named:" field (Windows 2000), type the name of the file you want to locate. Alternatively, in the "A word or phrase in the file" or the "Containing text:" field, you can enter search terms based on the file's content.
   • In the "Look in:" field, click the arrow to display a drop-down menu from which you can choose a drive or folder to search. Select the drive to search. To search all your hard drives and mapped network drives, choose My Computer.
   • Click Search (Windows XP), or Search Now (Windows 2000).
4. In the search results, find the .nk2 file with the name of the profile that you want to reset. Right-click the filename and select Rename.

For example, if your Outlook profile is named jdoe, right-click jdoe.nk2 and select Rename. Keep in mind that, unless you specifically named your Outlook profile after your username, you probably won't find yourusername.nk2 in the search. You will have to know your profile's name to identify the correct file in the search returns. In most cases, only one file will be found.  You can find your profile name by clicking on Start -> Control Panel -> Mail -> Show Profiles.

5. Change the file's extension to anything other than .nk2. For example, if the file you located is named jdoe.nk2, change it to something like jdoe.bak or jdoe.old. Restart Outlook. A new .nk2 file will automatically regenerate.