Problems with KMail Identities

I use a separate email address for handling mailing lists (the one this message is coming from) and in KMail I setup an Identity for it, however I can't post any messages to this list with it because KMail insists on re-writing the email with the "Return-Path" as my default email address -- and then the listserver rejects it saying I'm not subscribed. Is there anyway to get KMail to do the "Right Thing"?