Comment 1 for bug 266824

RFC2822 (Para 3.6.2. - Originator fields) would suggest that simply not
using
the Sender: header when Mailman is the agent responsible for the actual
transmission of the message is a breach of the RFC.

The problem with the behaviour of Outlook is discussed in the Mailman FAQ

here:
http://www.python.org/cgi-bin/faqw-mm.py?req=show&file=faq02.003.htp

A suggested code change which cosmetically improves the Outlook
presentation by changing the Sender: header value from the listname-
bounces alias to the listname alias is described there.

Paranthetically:

The Error-to: header is deprecated and only inserted for backward
compatibilty with old/broken MTAs.

The Return-path is not a header that travels with the message and is
known
as a trace field. It is mandatory and normally inserted at the start of
the
message, as a header, by the final SMTP delivery MTA and is the value of
the
SMTP envelope sender.

As an aside, Outlook 2003 appears to allow filtering on the contents of
From:
header wihtout any problems (as opposed to the From field displayed on the

GUI) and this does not involve the contents of the Sender: header.