function checkfields(){
	var str = "";

	if (document.contactf.naam.value=="") {
		str = str + " surname\n";
	}
	if (document.contactf.voorletters.value=="") {
		str = str + " first name\n";
	}	
	if (document.contactf.adres.value=="") {
		str = str + " address\n";
	}	
	if (document.contactf.postcode.value=="") {
		str = str + " postal code\n";
	}	
	if (document.contactf.woonplaats.value=="") {
		str = str + " city\n";
	}	
	if (document.contactf.telefoon.value=="") {
		str = str + " telephone\n";
	}			
	if (document.contactf.email.value=="") {
		str = str + " email\n";
	}else if ( email.value.length > 0 && ( email.value.indexOf("@",0) == -1 || email.value.indexOf(".",0) == -1 ) ) {
			str += " a correct email address\n"	;
	}
			
	 if(str == ""){
		  document.contactf.action = "form_contact.php";
		  document.contactf.submit();
	 }
	 else {
			alert("please fill in the following fields:\n" + str);
	 }
}