John A. Martin writes:
In the absence of a Message-ID on an outgoing mail message many if not most MTAs will add one. Why not let Mailman anticipate the need to add a Message-ID when archiving the message rather than leaving it to the outgoing MTA?
My reason for saying "last resort" is simply that this is not predictable to third parties. Eg, I send you (a non-subscriber) a message with CC and no Message-ID. You'd like to find the thread in the archives. You may as well just do a linear search on that month's threads.
An URL based on an MD5 of the message body in theory would work, but in the presence of non-ASCII bodies, structured MIME, ML digests, and various MTA autoconversions, that seems fragile.