code: def send(self, message, envelope_from=None):
          
          assert message.send_to, "No recipients have been added"
  
          assert message.sender, (
              "The message does not specify a sender and a default sender "
              "has not been configured")
  
          if message.has_bad_headers():
              raise BadHeaderError
  
          if message.date is None:
              message.date = time.time()
  
          ret = None
          if self.host:
              ret = self.host.sendmail(
                  sanitize_address(envelope_from or message.sender),
                  list(sanitize_addresses(message.send_to)),
                  message.as_bytes() if PY3 else message.as_string(),
                  message.mail_options,
                  message.rcpt_options
              )
  
          email_dispatched.send(message, app=current_app._get_current_object())
  
          self.num_emails += 1
  
          if self.num_emails == self.mail.max_emails:
              self.num_emails = 0
              if self.host:
                  self.host.quit()
                  self.host = self.configure_host()
  
          return ret
docs: Verifies and sends message.
  
          :param message: Message instance.
          :param envelope_from: Email address to be used in MAIL FROM command.