When testing out our mail server we stumbled accross an error that prevents us from sending mails via PHP, though regular sending/receiving per Mail-in-a-box works without any problems. We are running a separate Ubuntu 18.04 server that only has Mail-in-a-box with all its needed components running.
Output in the error.log text file
PHP Fatal error: Uncaught Swift_TransportException: Expected response code 354 but got code "554", with message "554 5.5.1 Error: no valid recipients "
PHP file
$request_email = $_POST['request-email'];
$request_name = $_POST['request-name'];
$request_text = $_POST['request-text'];
$transport = (new Swift_SmtpTransport('data.abc.xy', 587, 'tls'))
->setUsername('contact@abc.xy')
->setPassword('*******')
->setStreamOptions(array('ssl' => array('allow_self_signed' => true, 'verify_peer' => false)));
$mailer = (new Swift_Mailer($transport));
$message = (new Swift_Message('Name: '. $request_name))
->setFrom(['' . $request_email => '' . $request_name])
->setTo(['contact@abc.xy'])
->setBody('E-Mail: ' . $request_email . $request_text)
->setContentType("text/html");
$result = $mailer->send($message);
What we have tried is to reinstall all of Mail-in-a-box and all of the components and checking everything for spelling mistakes. The ricipient does exist on our mail server and can receive and send mails manually via the client.