Check out example codes for "Expected response code 250 but got code "501", with message "501 5.5.4 Invalid domain name". It will help you in understanding the concepts better.

Code Example 1

public function sendMessage()
    {

        $transport = (new \Swift_SmtpTransport('smtp.office365.com', 25, 'tls'))
            ->setUsername('[email protected]')
            ->setPassword('YourPassword');
        $transport->setLocalDomain('[127.0.0.1]');  //<= add this to your code
        $mailer = new \Swift_Mailer($transport);

        try {
            $message = (new \Swift_Message($this->Subject))
                ->setFrom(array('[email protected]' => "Subject"))
                ->setTo( $this->Email)
                //->setBcc($client)
                ->setBody($this->text, 'text/html');
            $mailer->send($message);

        } catch (\Exception $e) {
            return $e->getMessage();
        }

        return 1;

    }

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