I'm thinking of adding follow up pitches for other products to the website I am putting together for one product. With this, I'd like to give the opt-in subscriber the ability to specify how often he or she would like to hear from me. I.e. every 7 days, every 30 days, etc (or "weekly" "monthly" quarterly," etc.) Do you know of any auto-responders/list managers that can allow this specification by the subscriber?

Or failing that, does anyone know what the median frequency choice is for such follow-ups?

Thanks in advance for any help you can give me.