I have written a PHP autoresponder script that is almost ready to ship. If you are interested in obtaining a free copy, you can subscribe now to the email notification list. By subscribing you will be one of the first users of the system so it's possible that there may be some bug that I didn't notice.
I am busy with activities this weekend, so don't have much time to package the code and write documentation. But if you get on the notification list, you will be one of the first people to get hold of the autoresponder when it is ready for download.
The code runs stand-alone on any PHP hosting or local host machine. Also, I will include a WordPress plugin for use with blogs.
Maybe it can replace your monthly subscription cost to a 3rd party autoresponder service?
The sending part uses an SMTP server such as you get with cPanel hosting.
You can send timed sequences of emails, and broadcast emails to your list. Also, the subscribe, confirm, and unsubscribe features are automated.