code: def _raise_error_if_disconnected(self) -> None:
          
          if (
              self.transport is None
              or self.protocol is None
              or self.transport.is_closing()
          ):
              self.close()
              raise SMTPServerDisconnected("Disconnected from SMTP server")
docs: See if we're still connected, and if not, raise
          ``SMTPServerDisconnected``.