Send return email to the user in workflow.

Hi Guys, I am trying to send email from workflow. When i assign item to user then sending mail with all details to that user. Once that user approves it, then i want to get email as "Item is approved". I tried to check the email notification events like "OnClosed","OnVote" etc, but i am not getting any emails even it is approved by assigned user. Can you please help me on it and suggest.   Thanks, Aaba.