App to send automatic email follow-ups

I want to develop a simple app that automatically send follow-up emails if the email is not replied, that can be run online using a subscription model. Users must be able to: 1 - Create an account by adding his email email account to the app (adding email, password, incoming and outgoing server details) 2 - Send email to one or multiple addresses from the app, using a text editor 3 - Option to select from 1 to 10 follow-up emails and the email content that each one will have 4 - Option to select the number of days that will pass from each follow up email (for example: followup1 after 5 days, followup2 after 15 days, etc.) 5 - The app will automatically send the preschedule follow up emails if no replies have been received 6 - The app will detect when an email reply is received and automatically stop any further email followups Im looking for a simple but nice application that can be run online using a subscription model: I must be able to set up 5 monthly payment plans and allow users to signup for a monthly plan that will allow to send X number of emails per month for $X paid by paypal. So the app must deal with the payments and registrations, and count the number of emails sent per user and allow usage while payment is received. Basically the the app must run the subscription model automatically. The app interface must be nice looking, well designed and very easy to use. I am looking for someone with experience, so please send me samples of your work and provide details about how you will do the job. I am a serious employer with good feedback and I expect the same from the freelancer.