function validaEmail() {
  if (!isValidEmail(document.boletim.email.value) ) {
      alert("E-mail incorreto");
      document.boletim.email.focus();
      return false; 
  }
  return true;
}

function isValidEmail(email) {
    if ( !allValidChars(email)) {  
        return false;
    }
    if (email.indexOf("@") < 1) { 
        return false;
    }
    else if (email.lastIndexOf(".") <= email.indexOf("@")) { 
        return false;
    }
    else if (email.indexOf("@") == email.length) { 
        return false;
    }
    return true;
}

function allValidChars(email) {
  var parsed = true;
  var validchars = "abcdefghijklmnopqrstuvwxyz0123456789@_.-";
  for (var i=0; i < email.length; i++) {
    var letter = email.charAt(i).toLowerCase();
    if (validchars.indexOf(letter) != -1)
      continue;
    parsed = false;
    break;
  }
  return parsed;
}