Website: https://web-crunch.com Tutorial: https://web-crunch.com/posts/email-subscription-workflow-ruby-on-rails
This is the source code of a 6-part guide on creating e-mail notifications with optional settings tailored to a given user's preferences. Each user can enable or disable their e-mail subscriptions and also opt out directly from an e-mail without needing to sign in.
In the guide I build a primative project management app with a task list.