actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb
The Message-ID as specified by rfc822 is supposed to be a unique identifier for that individual email. That makes it an ideal tracking token for debugging and forensics, just like X-Request-Id does for web request.
If an inbound email does not, against the rfc822 mandate, specify a Message-ID, one will be generated using the approach from Mail::MessageIdField.