Hello,
As surprising as it may seem, Message-ID is actually not a mandatory email field [1]. While most MTAs do add this field, some might not, and this will cause load_message.py to crash.
As a solution to this, when this field is missing, this patch:
- attempts to find a "Sent-Message-ID" header and use it as the Message-ID (a case I encountered when trying to import an old mbox)
- generates a new Message-ID if none exists, following (a simpler version of) [2].
I don't think this should be the responsibility of pglister. As you say, "most MTAs do add this field" -- and the solution is to configure the MTA to do this. We already rely on the MTA to get a lot of other important things right.