Not what was expected of them. I included "minimum-viable" in my original reply for a specific reason: to counter OP's lament that they lost out to a simpler solution.

If you are asked to implement X and instead you take extra time and come back with X+Y+Z, what have you done? Wasted time e.g. money. Companies really hate wasting money.

So if in your candidate project you demonstrate a propensity for bike-shedding any task, that's gonna be a big red flag.

It was expected that the candidate filled the blanks. He specifically mentioned he would fill those blanks with Y+Z, before he spent time on them.

In the real world, if the time he spent was deemed wasted time, that's management's fault.

They explicitly asked for a minimal, terminal-inspired email client for their Email Backend Engineer role. OP built a ton of infrastructure, created a generic web app that has no semblance of terminal inspiration as far as I can tell, and outsourced the backend to third party APIs.

Concrete and working? Technically, yes. This would have been an excellent submission for a different assignment and role. But it doesn't seem to suit the specifications for this one.