I just found out an explanation for Outlook’s irritating behaviour that I was working around back in January. It turns out that Sendmail has never followed the standard’s requirements for the Sender: header (it never adds or fixes it), so the developers of Outlook assumed that you would only see a Sender: header when the message had been transmitted via a mailing list. This makes the “from [list] on behalf of [author]” wording look relatively sensible. However, they should have used the bloody List-ID: which is supposed to be used for exactly this kind of thing.
What is worse is that Exim is out on a limb in this respect. Postfix used to follow the standard but was changed in 2000 to be bug-compatible with Sendmail. This is very irritating, because the standard behaviour is much more useful. Grr.