[Mailman-Developers] In-Reply-To vs. References

[Aahz]

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.)