title: PHP mail stopped working
body: <p>Some days ago when using <code>mail()</code> I had it working.</p>
  
  <p>But now it doesn't work. And I don't know what the problem is.</p>
  
  <pre><code>$to      = 'testmail@gmail.com';
  $subject = 'the subject';
  $message = 'hello';
  $headers = 'From: sender@gmail.com' . "\r\n" .
      'Reply-To: sender@gmail.com' . "\r\n" .
      'X-Mailer: PHP/' . phpversion();
  
  mail($to, $subject, $message, $headers);
  
  $mail_sent = @mail( $to, $subject, $message, $headers ); 
  echo $mail_sent ? "Mail sent" : "Mail failed";
  </code></pre>
  
  <p>It displays "Mail sent".</p>
  
  <p>I haven't touched anything in Apache or this code. I have tested the code in an empty PHP file with the same result. How can I debug this problem?</p>