One on one communication between Site Owner and client

Hello!

I'm looking for a plugin that allows for me to communicate with the clients on my website with an email like fashion. I'm looking for a one-to-one solution between owner and client that works after a user registers. Preferably using the same account as the Appointments+ plug-in.

My goal here is to create a one-on-one math tutoring site that allows me to book clients as well as communicate with them within the site. The scheduler will be used to book Skype sessions.

Thanks!