email - Unable to send mails in PHP contact form using 000webhost webmail

Nothing wrong with your html I tried it.
If you have chrome you can use the developer tool to debug and see if your requests are sent and if they are lending on the correct path :

In your mailer.php output the $_POST variable to make sure your data is landing there correctly,

echo "
";
    var_dump($_POST);
echo "
";

You may also need to validate your html :

  
Name:
  
  
E-mail:
  
  
Subject:

  
  
Message: