Building a data-driven approach to keeping users' email addresses up-to-date

Here's another thing you can do in your app: employ a custom frequency of periodic emails on a per-user basis. It doesn't help to keep emailing someone who never reads your stuff, they either do not need/like your product and ignoring you, or (and this is much worse) they're reporting your mailings as "spam". That, if left untreated, may affect the "spamicity score" of your domain: not good.

Post-author here. This was a fun sample app to build. Heroku scheduler made it really easy to deploy the invoice emails on a set schedule. If you have any feedback on ways to improve the app or if you would have done anything differently, let me know.

It would probably be better from a economic standpoint just to let your users keep using the email that they are used to using on your site.