I am at the tail end of building a forum/Q&A community-based application, and I would like to add email notifications. The app has several different entities, including: threads, questions, projects, photos, etc. The goal is that a user can "subscribe" to any number of these entities, queuing an e-mail whenever the entity receives new comments or activity. This functionality is very similar to facebook and forums.
I have looked into ActionMailer (with rake tasks and delayed jobs), MailChimp API (and plugins), and other app mailers (PostageApp and Postmark).
I am leaning against ActionMailer, because of potential issues with memory hogging and server overload. The app will be running on Heroku, but I'm afraid the servers could be easily overwhelmed sending out potentially hundreds of emails every few minutes.
Another complexity is that there will be different types of subscriptions (instant email notification, daily email notification) based on user preference.
What would be the best way to manage email for functionality like this? Any tips/ideas are greatly appreciated!