docs: def verify(self):
          '''Verifies the message data based on rules and restrictions defined
          in the Postmark API docs.  There can be no more than 20 recipients
          in total. NOTE: This does not check that your attachments total less
          than 10MB, you must do that yourself.
          '''
          if self.to is None:
              raise MessageError('"to" is required')
          if self.html is None and self.text is None:
              err = 'At least one of "html" or "text" must be provided'
              raise MessageError(err)
          self._verify_headers()
          self._verify_attachments()
          if (MAX_RECIPIENTS_PER_MESSAGE and
                  len(self.recipients) > MAX_RECIPIENTS_PER_MESSAGE):
              err = 'No more than {0} recipients accepted.'
              raise MessageError(err.format(MAX_RECIPIENTS_PER_MESSAGE))