We've been having a discussion on comp.lang.py about Mailman's handling of headers in the context of (not) threading messages, and Barry Warsaw suggested that I continue the discussion here.
Summary: I propose that in the mail-to-news gateway (but not the other direction), if the In-Reply-To: header exists but the References: header does not, copy In-Reply-To to References.
Background:
In a large fraction of e-mail programs, the In-Reply-To header contains the Message-ID of the message being responded to. According to RFC822, both In-Reply-To and References are valid for this purpose. However, newsreaders only recognize the References header (RFC1036). The absence of a References header frequently breaks the ability of newsreaders to thread messages together. (Most newsreaders can still do Subject-based grouping, but that's not the same thing as threading.)
Much of the discussion on comp.lang.py has revolved around the fact that it's a Bad Idea to muck around with headers (a philosophy I generally agree with). However, it seems to me that in this case there is a clear translation issue between the domain of e-mail messages and the domain of newsgroup postings (as specified by the appropriate RFCs) and that the potential negative consequences are minimal. (I foresee some possible oddities if someone replies to a private e-mail message and cc's the mailing list -- but that problem already exists if an e-mail program uses References instead of In-Reply-To.)