Thank you! I'm envisioning a future where email is a web application framework, so that the internet as a network of machines becomes a background detail, and application development happens at a higher level--on a protocol for a network of people. For example, if you're a developer and you want to be able to do something in your email conversations that you can't already do, you could write a plugin that performs that task (assuming you couldn't find a pre-existing plugin that did what you want), and then you could use it with everyone that you can communicate with via email because it would be directly integrated into your conversations. No server-side coding would be necessary, and you wouldn't have to bother with getting people to use your application because it would already be in a convenient place for them.

Coming back to your question: I see the potential use cases for this as primarily a superset of the current use cases for email. For bulk mailing and automated notifications, SMTP remains superior by nature of its simplicity (mostly... though this application edits self-generated notification messages to append notifications when there are multiple in a short time period). Email clients for personal use tend to have an immense number of bells and whistles, but if you assume that those will inevitably creep into this application as well over time, it should eventually be wholly practical for personal email.

Business emailing needs can be significantly more specialized, but they're also more likely to want advanced capabilities from their communication software. Assuming that some additional plugins offering desirable features are created, this could be very useful in a business setting.

I've added some gifs to the README that show editing, adding an inline reply, and adding a plugin. I know the flow to add a plugin is pretty gnarly at this point. I'm planning to pull them more directly into message content in the near future, because I tend to forget they're there when I have to click a button to even see that area. :)