function checkSubmit() {
    var companyName = document.getElementById('companyName').value;
    var contactPerson = document.getElementById('contactPerson').value;
    var email = document.getElementById('email').value;
    var telNumber = document.getElementById('telNumber').value;

    if (companyName == '' || companyName == 'bedrijfsnaam *') {
    alert('U dient uw bedrijfsnaam in te vullen!');
    return false;    
    }
    
    if (contactPerson == '' || contactPerson == 'contactpersoon *') {
    alert('U dient uw contactpersoon in te vullen!');
    return false;    
    }
    
    if (!email.match(/^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i)) {
    alert('Ongeldig e-mailadres!');
    return false;                                    
    }    
    
    if (telNumber == '' || telNumber == 'telefoon *') {
    alert('U dient uw telefoonnummer in te vullen!');
    return false;    
    }
}