Help me help the helpful!

I work at a non-profit and I have a group of volunteers to coordinate. This actually can eat up a chunk of my time. I already know that tasks that need to be done and I need a way to send out timed reminders to volunteers.

I want to be able to input all the future reminders at one time and trust that at the appointed time in the future, the volunteers will be duly reminded.

Barriers: volunteers use a host different e-mail services and also do not want to sign up for any more lists/services.

I have found many services that will remind one person but haven't found any that will remind others without their subscription to the service.

Can anyone help me with a suggestion?