Hi genkiz,
I don’t have the ultimate solution for you, but I can at least say that you will be getting into some things you didn’t expect.
I think you could handle the code and logic of what, when, and how people are supposed to receive e-mails… stuff that would happen in Codeigniter.
Then you need the server to constantly or periodically check if the application should send e-mail reminders out to anyone. This may involve cron jobs at the server level:
http://www.aota.net/Script_Installation_Tips/cronhelp.php3
Once you got that taken care of, you may have issues with the volume of mail going out and have to send it in queued batches. Hopefully you don’t have to deal with your server getting blacklisted as a spam server, etc. These are all some things to think about at least and show that your solution may not be completely in CI.
EDIT: I just saw this post and may be another thing to check out with cron jobs and CI:
http://codeigniter.com/forums/viewthread/108154/