Computer Program that reminds you to send a letter?

Ok, at my job we have to send out letters to our newest inquiries. 2 weeks later we are supposed to send out another letter to those same people and then quarterly we have to send out a newsletter to them, Forever, until they die lol.. so anyways.. I was wondering if there is some sort of program that will pop up with the addresses we would have inserted into it 2 weeks later or even 3 months later. Right now we are creating a folder inside different agents folders and it will eventually become to big and too much of a hle. Sorry if my question is confusing if you need me to clarify more please let me know. Thank you!!