When we originally built the TogetherLetters app, we just hacked together an email templating solution. That was a huge mistake. We have now transitioned over to a standardized version using an add-on that lets us put some of the generic logic into our email templates. This should help us moving forward in implementing features much more easily and making sure emails reflect the proper information to the correct audiences.
In addition, we're now including fields that tell people about the group they are getting emails for and potentially what kinds of updates are being solicited. They also know when the deadline is for their updates but hopefully, this causes a sense of urgency to click and provide an update instead of letting people procrastinate. Time will tell on this!