Outlook 07

I have a user with Office 2007 and they have created a distribution list in Outlook. An employee left and was removed from the distribution list. The name does not show in the list and I have removed them from our server. When the user emails to the distribution list it sends back a reply saying that the user we removed from the list is unreachable. It is still trying to email the user we removed from the list. Where is it holding this name that was supposedly removed from the list?