review-submit does not set Message-Id on outgoing emails

[James Henstridge]

Tim sent me a review-submit email which was also CC'd to the reviews mailing list. I received two copies of the message: one with a message ID added by my MTA and one with a message ID added by a canonical.com MTA.

While my mail client usually does duplicate suppression, it displayed both messages separately because they had different message IDs. The review-submit plugin should be adding its own message IDs rather than relying on intermediate MTAs to do so. If it did this, then I'd only have seen one copy of the email.

The email.utils.make_msgid() function can be used to generate message IDs.