Zimbra "Send As" and Outlook

[knabe]

I have configured the "Send As" feature for a user, who wants to send email "from" a Mailman list, so that when people respond to the email it goes back to the list. It is working great from the Web Interface, but they use the Outlook connector. When they send from Outlook, they get an error that the user does not exist on the server. Is there an Outlook configuration that I am missing. It is Outlook 2003, and Zimbra 5.0.15.