Outlook has an automatic function to suggest email addresses as you start to type them in. This works in the "to", "CC" and "bcc" (blind copy) fields. This handy time saver uses the list of your previously used email addresses and saves time compared to looking up the name in your address book.

A problem arises when somone changes their email address or otherwise falls out of your favor. If the auto-complete becomes a total nuisance, you can turn it off (Tools -> Options -> EMail Options -> Advanced). Slightly less dire, you can periodically clear the cache and start over, by navigating to C:\Documents and Settings\{your WIndows log in name}\Application Data\Microsoft\Outlook and deleting all files with the extension nk2.

What if you just want to eliminate one or two out of date or out of fashion names? You would think you could just open up the nk2 file and edit it. That would be nice, but like other such MS Office things, the file is binary and not editable.

What you can do is, from within Outlook, as the program starts to suggest a email names, highlight the one you don't want anymore and press Delete. That will remove it from the cache.

(Other email program have similar caching functions; consult documentation for managing them.)