php form doesn't send

When I submit my form it Dies for an unkown reason.

My form code:

    <form action="mail.php" method="post">
        <input class="invoeren" name="naamUser" type="text" placeholder="Naam" required>
        <input class="invoeren" name="nummerUser"  type="text" placeholder="Telefoonnummer" required>
        <input class="invoeren" name="onderwerpUser"  type="text" placeholder="Onderwerp" required>
        <textarea class="invoeren" type="text" name="opmerkingUser"  placeholder="Opmerking" required></textarea>           
        <button type="submit" id="contactButton">Verstuur</button>
    </form>

My mail.php file:

<?php 
    ob_start(); 

    $naamUser = $_POST['naamUser'];
    $nummerUser = $_POST['nummerUser'];
    $onderwerpUser = $_POST['onderwerpUser'];
    $opmerkingUser = $_POST['opmerkingUser'];


    $formcontent="Aanvraag formulier Domein naam \n
    Naam: $naamUser \n
    Telefoonnummer: $nummerUser \n
    Onderwerp: $onderwerpUser \n
    Opmerking: $opmerkingUser \n              
                  ";              

    $recipient = "email@email.nl";

    $subject = "Terug Bellen";

    $mailheader = "From: $naamUser \r\n";

    mail($recipient, $subject, $formcontent, $mailheader) or die("Error!");
    ?>

I use 1 mail.php file for several index.html files, is that maybe the reason why it dies?