RSVP lets you setup reminders for important sent messages, and the reminders trigger when you do not get replies to those mails. This way, you can follow up with recipients of your mails and track down important tasks once sufficient time has elapsed. It also lets you setup reminders for having received a reply to an e-mail and more importantly for not having received a reply.
The coolest feature of the app is that once a reminder is set, you no longer need to save the original sent e-mail. The app saves the sent e-mail and can be accessed at anytime by clicking on the subject hyperlink through the Manage Reminders section.