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There are several reasons why messages could be returned to you as undeliverable. Perhaps the email address is incorrect or no longer exists.
Another reason is that the Outlook AutoComplete/Nickname cache has become corrupt. If the AutoComplete/Nickname cache file becomes corrupt, Outlook may not be able to identify recipients, may offer incorrect recipients, may send to an incorrect or old email address, or may send the message to the wrong person. In this case, you can clear the entire cache. Please click here and follow steps that are appropriate for your version of Microsoft Windows to clear the AutoComplete/Nicknames cache. After you restart Outlook, Outlook generates a new nickname list.
