Triggering an Apex method with a Custom Button

Hi,

I have a custom button in account called 'submit' and it's a onclick Java script event when i click on the button it displays error

A problem with the OnClick JavaScript for this button or link was encountered:
{faultcode:'soapenv:Client', faultstring:'No operation available for request {http://soap.sforce.com/schemas/package/OutboundEmails}SendEmailNotification, please check the WSDL for the service.', }

iam using 21.0 version

JS code:

{!REQUIRESCRIPT("/soap/ajax/

21.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/21.0/apex.js")}

var AccId= '{!Account.Id}';
sforce.apex.execute("OutboundEmails","SendEmailNotification", {id:AccId});
window.alert("Account Id is sent." )

Apex code:

global class OutboundEmails {
WebService static void SendEmailNotification(Id id) {

//create a mail object to send a single email.
Messaging.SingleEmailMessage mail = new
Messaging.SingleEmailMessage();
//set the email properties

mail.setToAddresses(new string[] {'Myself@gmail.com'});

mail.setSenderDisplayName('SF.com Email Agent');
mail.setSubject('A new reminder');
mail.setHtmlBody('an object with ID='+ id + ' is just clicked on.');
//send the email
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail } );

}
}

ERROR:sforce is not defined