function isValidEmail (emailToTest) {
  var message='';
  var result=true;
  if (emailToTest=='') {
      message = "You have not given your email address.";
      result=false;
  } else if (noAtSign( emailToTest )) {
      message = "The email address \"" + emailToTest + "\" does not contain an '@' character.";
      result=false;
  } else if (nothingBeforeAt(emailToTest)) {
      message = "The email address \"" + emailToTest;
      message += "\" must contain at least one character before the '@' character";
      result=false;
  } else if (noLeftBracket(emailToTest)) {
      message = "The email address \"" + emailToTest;
      message += "\" contains a right square bracket ']',\nbut no corresponding left square bracket '['.";
      result=false;
  } else if (noRightBracket(emailToTest)) {
      message = "The email address \"" + emailToTest;
      message += "\" contains a left square bracket '[',\nbut no corresponding right square bracket ']'.";
      result=false;
  } else if (noValidPeriod(emailToTest)) {
      message = "The email address \"" + emailToTest + "\" must contain a period ('.') character.";
      result=false;
  } else if (noValidSuffix(emailToTest)) {
      message = "The email address \"" + emailToTest;
      message += "\" must contain a two, three or four character suffix.";
      result=false;
  } 
  return (result);
}

function noAtSign (formField) {
    // CHECK THAT THERE IS AN '@' CHARACTER IN THE STRING
    return (formField.indexOf ('@', 0) == -1);
}

function nothingBeforeAt (formField) {
    // CHECK THERE IS AT LEAST ONE CHARACTER BEFORE THE '@' CHARACTER
    return ( formField.indexOf ( '@', 0 ) < 1 );
}

function noLeftBracket (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN CHECK FOR LEFT BRACKET
    return ( formField.indexOf ( '[', 0 ) == -1 && formField.charAt (formField.length - 1) == ']');
}

function noRightBracket (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN CHECK FOR RIGHT BRACKET
    return (formField.indexOf ( '[', 0 ) > -1 && formField.charAt (formField.length - 1) != ']');
}

function noValidPeriod (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN WE ARE NOT INTERESTED
    if (formField.indexOf ( '@', 0 ) > 1 && formField.charAt (formField.length - 1 ) == ']')
        return ( false );

    // CHECK THAT THERE IS AT LEAST ONE PERIOD IN THE STRING
    if (formField.indexOf ( '.', 0 ) == -1)
        return ( true );

    return ( false );
}

function noValidSuffix(formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN WE ARE NOT INTERESTED
    if (formField.indexOf('@', 0) > 1 && formField.charAt(formField.length - 1) == ']') {
        return ( false );
    }

    // CHECK THAT THERE IS A TWO OR THREE CHARACTER SUFFIX AFTER THE LAST PERIOD
    var len = formField.length;
    var pos = formField.lastIndexOf ( '.', len - 1 ) + 1;
    return ( ( len - pos ) < 2 || ( len - pos ) > 4 );
}