Form mail doesn't send when not all fields are filled in

[eLIX]

Hi,

I'm pretty new to PHP and I'm working on a PHP contact form.
Now everything works fine if all the fields are filled in. But when there's one field that's not filled in, the mail doesn't send.
Here's my code:

[code]
<?php
$zender_geslacht = $_POST['geslacht'];
$zender_naam = $_POST['contactpersoon'];
$zender_bedrijf = $_POST['bedrijf'];

$zender_adres_straat = $_POST['adresStraat'];
$zender_adres_nummer = $_POST['adresNummer'];
$zender_adres_postcode = $_POST['adresPostcode'];
$zender_adres_gemeente = $_POST['adresGemeente'];

$zender_contact_tel = $_POST['contactTel'];
$zender_contact_fax = $_POST['contactFax'];
$zender_contact_email = $_POST['contactEmail'];

$zender_aantalpersonen = $_POST['aantalPersonen'];
$zender_bericht = $_POST['vragen_opmerkingen'];

$zender_email = $_POST['contactEmail'];

$to      = '[email protected]';
$subject = 'Luchtballon.be Aanvraag';

$message = $zender_geslacht . ' ' . $zender_naam . "\r\n" .
'Bedrijf: ' . $zender_bedrijf .
"\r\n" . "\r\n" .
'Straat: ' . $zender_adres_straat . "\r\n" .
'Nummer: ' . $zender_adres_nummer . "\r\n" .
'Postcode: ' . $zender_adres_postcode . "\r\n" .
'Gemeente: ' . $zender_adres_gemeente .
"\r\n" . "\r\n" .
'Telefoon: ' . $zender_contact_tel . "\r\n" .
'Fax: ' . $zender_contact_fax . "\r\n" .
'Email: ' . $zender_contact_email .
"\r\n" . "\r\n" .
'Aantal Personen: ' . $zender_aantalpersonen .
"\r\n" . "\r\n" .
'Bericht: ' . $zender_bericht;

$headers = 'From: ' . $zender_email . "\r\n" .
  'Reply-To: ' . $zender_email . "\r\n" .
  'X-Mailer: PHP/' . phpversion();

mail($to, $subject, $message, $headers);
?>
[/code]