function validateForm() {
    if(document.getElementById('name').value == '') {
        alert('The field "name" is required.');
        return false;
    }
    if(document.getElementById('email').value == '') {
        alert('The field "e-mail" is required.');
        return false;
    }
    if(document.getElementById('subject').value == '') {
        alert('The field "subject" is required.');
        return false;
    }
    if(document.getElementById('message').value == '') {
        alert('The field "message" is required.');
        return false;
    }

    return true;
}