<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def hash_key(self):
 """ Returns the canonical hash of a mail. """

        if self.conf.message_id:
            message_id = self.message.get('Message-Id')
            if message_id:
                return message_id.strip()
            logger.error(
                "No Message-ID in {}: {}".format(self.path, self.header_text))
            raise MissingMessageID

        return hashlib.sha224(self.canonical_headers).hexdigest()