docs: def header(self, name, value):
          """
          Store all message headers, optionally clean them up.
  
          This simply stores all message headers so we can send them to DSPAM.
          Additionally, headers that have the same prefix as the ones we're
          about to add are deleted.
  
          """
          self.message += "{}: {}\r\n".format(name, value)
          logger.debug('<{}> Received {} header'.format(self.id, name))
          if name.lower().startswith(self.header_prefix.lower()):
              self.remove_headers.append(name)
              logger.debug('<{}> Going to remove {} header'.format(
                  self.id, name))
          return Milter.CONTINUE