I get similar results when using mutt... I'm not sure this is completely related to Outlook either. I guess I just have to keep looking.
I'm not sure what you're seeing with mutt, but what I see is if From: contains a 'real name' I am shown the real name and not the email address in the 'index', but when I view the message, I see the actual From: header and nowhere do I ever see 'from the sender on behalf of ...'.
The bottom line is Mailman doesn't alter the From: header (except in the case of an anonymous list).
Mailman does alter the Sender: which seems to be in accord with the RFCs as Mailman is the agent that sends the message to the list members.
-- Mark Sapiro msapiro@value.net The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan