I have idea how to send e-mails (some serie, course or something like this)
based on RSS e-mailes !
1. one rss message is send only ONE time to one email
2. Shedulning of sending e-mails i defined (Daily, weekly...)
3. Reque some messeges by cron RSS messeges every day starting from messege nr1 to nrX.
User will get firs message in queue which he/she didn't get before. Only one because shedulnge is defined.
Offcourse it's not stright way to do it but should work.
Maybe I'm wrong. I didn't test it because I need days