function isInput(veld) {
	if (veld == null || veld == "" || veld == veld.defaultValue) {
		return false;
	}
	return true;
}

function isEmail(f) {

    if (f.value.search(/^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/) != -1)
  	{
        return true;
    }
	else
	{
		f.focus();
        return false;
    }
}

function CheckMailAFriend() {
	msg = "";

	if (!isInput(document.forms.formulier.txtSenderName.value)) { msg += "Vul a.u.b. uw naam in.\n";}
	
	if (!isInput(document.forms.formulier.txtSenderEmail.value)) {
		 msg += "Vul a.u.b. uw e-mailadres in.\n";
	}
	else {
		 if (!isEmail(document.forms.formulier.txtSenderEmail)) msg += "Uw e-mailadres is niet correct.\n";
	}
	
	if (!isInput(document.forms.formulier.txtRecipientEmail.value)) {
		 msg += "Vul a.u.b. het e-mailadres van uw vriend in.\n";
	}
	else {
		 if (!isEmail(document.forms.formulier.txtRecipientEmail)) msg += "Het e-mailadres van uw vriend is niet correct.\n";
	}
	
	if (!isInput(document.forms.formulier.txtSubject.value)) { msg += "Vul a.u.b. een onderwerp in.\n";}
	
	if (msg == "") {
		document.forms.formulier.submit(); 
	}
	else {
		alert(msg);
	}
}