Addition to custom email code

I have a custom email code on my site that sends a pre-written reply email message to anyone who emails an account on my server. The code keeps a database of all incoming email addresses. If the same email address sends any additional emails to the same email account on my server, their email does not receive any future replies. This is how the code currently functions and it is working good. I would like this changed. Instead, I would like the ability to add an unlimited number of follow up emails. Each with its own custom subject line. (Or if I leave the subject line blank, it will use the original subject line with the "Re: ") I need to also able to determine the amount of days and hours that pass before each email reply is sent. For example: Right now the first reply is sent after a 10 minute delay. I would like the ability to have the second email sent 4 hours later. Then the third email sent 24 hours after the first email was sent. Then then fourth email sent 48 hours after the first email was sent and so on. . . The email sequence will continue to be sent to their email address until the server receives a second email from the same email address. The contents of the second email do not matter. But once a second email is received, all future emails stop. No future email replies are sent to the user after the second email is received, including the initial email reply.