code: async def expn(
          self, address: str, timeout: DefaultNumType = _default
      ) -> SMTPResponse:
          
          await self._ehlo_or_helo_if_needed()
  
          parsed_address = parse_address(address)
  
          async with self._command_lock:
              response = await self.execute_command(
                  b"EXPN", parsed_address.encode("ascii"), timeout=timeout
              )
  
          if response.code != SMTPStatus.completed:
              raise SMTPResponseException(response.code, response.message)
  
          return response
docs: Send an SMTP EXPN command, which expands a mailing list.
          Not many servers support this command.
  
          :raises SMTPResponseException: on unexpected server response code