the best one :D (RFC-friendly & no error "too complex") :
function isMail(mail)
{
pattuser = /^([A-Z0-9_%+\-!#$&'*\/=?^`{|}~]+\.?)*[A-Z0-9_%+\-!#$&'*\/=?^`{|}~]+$/i;
pattdomain = /^([A-Z0-9-]+\.?)*[A-Z0-9-]+(\.[A-Z]{2,9})+$/i;
tab = mail.split("@");
if (tab.length != 2)
return false;
return (pattuser.test(tab[0]) && pattdomain.test(tab[1]));
}