Yesterday I wrote this post to explain how products use emails as interfaces. I've found 4 use cases (Basecamp, Asana, JIRA, Linkedin & Twitter). Do you have others?
"For years emails was seen as a way to deliver messages. Today the line between products and emails is blurring. We are now seeing a silent shift in this approach where emails are not just a medium to deliver valuable information but a way to extend product (core in some cases) functionalities and give users a secondary way to interact with products."