function checkValidation(formInput) {
 
    if (typeof(formInput) != "object") {
        alert("No es posible validar el formulario con su navegador.");
        return(false);
    }
 
    var message;
 
    if (stringEmpty(formInput.value)) {
        message = "Error: No ha introducido ningún valor en el campo Email.";
        alert(message);
        return(false);
    } else if (noAtSign( formInput.value )) {
        message = "Error: La dirección de correo \"" + formInput.value + "\" no contiene el carácter '@'.";
        alert(message);
        return(false);
    } else if (nothingBeforeAt(formInput.value)) {
        message = "Error: La dirección de correo \"" + formInput.value;
        message += "\" debe contener al menos un carácter antes de la '@'";
        alert(message);
        return(false);
    } else if (noLeftBracket(formInput.value)) {
        message = "Error: La dirección de correo \"" + formInput.value;
        message += "\" contiene un ']',\npero no el correspondiente '['.";
        alert(message);
        return(false);
    } else if (noRightBracket(formInput.value)) {
        message = "Error: La dirección de correo \"" + formInput.value;
        message += "\" contiene un '[',\npero no el correspondiente ']'.";
        alert( message);
        return(false);
    } else if (noValidPeriod(formInput.value)) {
        message = "Error: La dirección de correo \"" + formInput.value + "\" debe contener un carácter ('.').";
        alert(message);
        return(false);
    } else if (noValidSuffix(formInput.value)) {
        message = "Error: La dirección de correo \"" + formInput.value;
        message += "\" debe contener un sufijo de 2, 3, ó 4 caracteres";
        alert(message);
        return(false);
    } else {
        return (true);
    }
 
    var objType = typeof(formInput.focus);
    if (objType == "object" || objType == "function") {
         formInput.focus();
    }
 
 
}
 
function checkValid (Email) {
    if ( checkValidation ( Email ) == true ) {
		return( true );
    }
	else {
 	   return ( false );
 	}
}
 
function stringEmpty (formField) {
    // CHECK THAT THE STRING IS NOT EMPTY
    if ( formField.length < 1 ) {
        return ( true );
    } else {
        return ( false );
    }
}
 
function noAtSign (formField) {
    // CHECK THAT THERE IS AN '@' CHARACTER IN THE STRING
    if (formField.indexOf ('@', 0) == -1) {
        return ( true )
    } else {
        return ( false );
    }
}
 
function nothingBeforeAt (formField) {
    // CHECK THERE IS AT LEAST ONE CHARACTER BEFORE THE '@' CHARACTER
    if ( formField.indexOf ( '@', 0 ) < 1 ) {
        return ( true )
    } else {
        return ( false );
    }
}
 
function noLeftBracket (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN CHECK FOR LEFT BRACKET
    if ( formField.indexOf ( '[', 0 ) == -1 && formField.charAt (formField.length - 1) == ']') {
        return ( true )
    } else {
        return ( false );
    }
}
 
function noRightBracket (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN CHECK FOR RIGHT BRACKET
    if (formField.indexOf ( '[', 0 ) > -1 && formField.charAt (formField.length - 1) != ']') {
        return ( true );
    } else {
        return ( false );
    }
}
 
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;
    if ( ( len - pos ) < 2 || ( len - pos ) > 4 ) {
        return ( true );
    } else {
        return ( false );
    }
}