function check_mail(s){
	var err = "La dirección de e-mail introducida es incorrecta";
	var digits = "0123456789";
	var lowercaseLetters = "abcdefghijklmnopqrstuvwxyz";
	var uppercaseLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	var validDomainNameChars = digits + uppercaseLetters + lowercaseLetters + "-_.";
	var i = 1;
    var sLength = s.length;
    while ((i < sLength) && (s.charAt(i) != "@")){
		i++
	}
    if ((i >= sLength) || (s.charAt(i) != "@")){
		alert (err);
		return false;
	}else{
		atloc = i;
    	j = i+1;
    	i += 1;
	}
	
    while ((j < sLength) && (validDomainNameChars.indexOf(s.charAt(j)) != -1)){
		j++
	}
	
    if (j < sLength){
		alert(err+s.charAt(j));
		return false;
	}
	
    while ((i < sLength) && (s.charAt(i) != ".")){
		i++
	}
	
    if (i == sLength){
		alert(err);
		return false;
	}
	
    if (i == (atloc +1)){
		alert(err);
		return false;
	}
	
    k = atloc+1;
	
    while (k < sLength){
		if ((s.charAt(k) == ".") && (s.charAt(k+1) == ".")){
			alert(err);
			return false;
		}
		k++
    }
	
    l = sLength;
	
    while ((i < sLength -2) && (l != i) && (s.charAt(l) != ".")){
		l = l-1
	}
	
    if ((i >= sLength - 2) || (s.charAt(i) != ".") || (l >= sLength - 2)){
		alert(err);
		return false;
	}else{
		return true;
	}
}