Hi Mahua,
It has not been implemented yet.
If you would like to pay/sponsor me to implement it then I would be glad to make it a priority and discuss how it should be done. For example should notification happen immediately when the subscribe form is submitted or should it only happen after the subscription has been confirmed by clicking the link in the email sent to the subscriber.
If you don't want to sponsor me financially to do it then the only quick and dirty solution I can suggest is to implement it yourself in the database using an insert trigger on the mp_LetterSubscribe table.
Best,
Joe