function echeck(str) {
    var at="@"
    var dot="."
    var lat=str.indexOf(at)
    var lstr=str.length
    var ldot=str.indexOf(dot)

    if (str.indexOf(at)==-1){
       return false
    }

    if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
       return false
    }

    if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
        return false
    }

     if (str.indexOf(at,(lat+1))!=-1){
        return false
     }

     if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
        return false
     }

     if (str.indexOf(dot,(lat+2))==-1){
        return false
     }

     if (str.indexOf(" ")!=-1){
        return false
     }

     return true
}

function guestbook_submit() {

    var form = document.getElementById('guestbookform');
    var name = document.getElementById('name');
    var mail = document.getElementById('mail');
    var text = document.getElementById('text');

    if (!name.value.length) {
        alert("Insert your name!");
        return;
    }

    if (!echeck(mail.value)) {
        alert("Insert a valid e-mail address!");
        return;
    }

    if (!text.value.length) {
        alert("Insert a message!");
        return;
    }

    if (text.value.length > 300) {
        alert("Your message was more than 300 characters long and has been cut.\nIf you're ok with it push 'sign' again, otherwise edit it.");
        text.value = text.value.substring(0, 299);
        return;
    }

    form.submit();
}