[pikzen]

... Then make an endpoint where you can get a task's status through its ID (your post to sendmail would then return a task ID), and poll repeatedly until it's marked as done, wasting bandwidth, your architecture, your choices ¯\_(ツ)_/¯

Or you can let the server notify you when it's done, and carry on with your work. As a bonus, most clients able to receive SSE already include automatic reconnection to the feed if it ever gets cut.

[icebraining]

They're not incompatible, though. I'd make everything as a REST endpoint (since you need to create a resource with an ID anyway for SSE, otherwise, how will the client know which email want sent?), and then add SSE as an optional performance improvement layer on top of those endpoints that may benefit from it.

This is how WebSub (formely PubSubHubbub) works on top of RSS/Atom, for example.