Swiftmailer一直在失败，我不明白为什么

Question

根据我的检查和die，它一直失败。有什么理由吗？ SMTP设置正确...

require_once '../lib/swift_required.php';

// Create the Transport the call setUsername() and setPassword()
$transport = Swift_SmtpTransport::newInstance('smtp.server.com', 25)
  ->setUsername('email@server.com')
  ->setPassword('password')
  ;

// Create the Mailer using your created Transport
$mailer = Swift_Mailer::newInstance($transport);


// Creating a test message
$message = Swift_Message::newInstance('Wonderful Subject')
  ->setFrom(array('john@doe.com' => 'John Doe'))
  ->setTo(array('receiver@domain.org', 'other@domain.org' => 'A name'))
  ->setBody('Here is the message itself')
  ;

// Send the message
$numSent = $mailer->send($message);

if ($mailer->send($message))
{
  die('Mail sent');
} else {
  die('Error: Mail failed');
}