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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.

Methods
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Instance Public methods
create_and_extract_message_id!(source, **options)

Create a new InboundEmail from the raw source of the email, which is uploaded as an Active Storage attachment called raw_email. Before the upload, extract the Message-ID from the source and set it as an attribute on the new InboundEmail.

# File actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb, line 16
def create_and_extract_message_id!(source, **options)
  message_checksum = Digest::SHA1.hexdigest(source)
  message_id = extract_message_id(source) || generate_missing_message_id(message_checksum)

  create! options.merge(message_id: message_id, message_checksum: message_checksum) do |inbound_email|
    inbound_email.raw_email.attach io: StringIO.new(source), filename: "message.eml", content_type: "message/rfc822"
  end
rescue ActiveRecord::RecordNotUnique
  nil
end
extract_message_id(source)
# File actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb, line 28
def extract_message_id(source)
  Mail.from_source(source).message_id rescue nil
end
generate_missing_message_id(message_checksum)
# File actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb, line 32
def generate_missing_message_id(message_checksum)
  Mail::MessageIdField.new("<#{message_checksum}@#{::Socket.gethostname}.mail>").message_id.tap do |message_id|
    logger.warn "Message-ID couldn't be parsed or is missing. Generated a new Message-ID: #{message_id}"
  end
end