I offer a service to my clients by email for which they pay me an amount for 30 days. I offer them the service for 30 days against one payment. For example, A pays me on the 2nd of Feb, B pays me on the 17th of March and C pays me on the 27 of March. The email is common to all, i.e. the same email goes to whoever is my client on a given date. So, in this case I will be sending the email to A from 2nd Feb to 2nd march, to B from 17 March to 17 April and to C from 27 March to 27th April. I need a tool that will give me a list of email IDs to whom I am supposed to send the email on a given date. Whenever someone pays me, I shall extend his validity by 30 days and so on. Is such a tool available. A free one would obviously be preferred. Please help. Thank you.