(OP outsourced the actual "email backend", that is the thing that stores and sends/receives emails, to a third-party products. Probably not the wisest solution if you are trying to get hired to work on email backends :) )
Just reading the thread is obvious that the requirements were sufficiently vague -- which, again, is not necessarily a problem -- but going about this in a very disrespectful way is not.
(And yes, of course, the hiring manager always can say that "oh I did not want to reject them before they sent the finished assignment, because they could have surprised us with their code!" -- which, while technically true, but I think simply showcases the absolute uncaring laziness that we see from many companies.)
> The UI will be kept simple, showing pagination for sent and received emails. In addition to the requirements of the assessment, there will be a login screen ...
How would you reply to that?
One option is to tell them that the document looks fine, but also add that it does not actually describe the parts they are going to grade on (such as: "which part of aerc/mutt is does author imitate?"). But this is a bad idea, because the likely reaction of candidate is to spend even more time on spec... and the assignment is not about writing specs, the specs are not in the rubric, and you don't want to waste candidate's time on asking for docs you don't care about.
Another option is to tell them: "This spec is so bad, I don't think you can possibly pass. Bye-bye!". This is even worse, as it can potentially reject good candidates who are just bad at writing documents - and there are plenty of them.
So I think responding: "Looking forward to receiving your submission." is the pretty OK answer.
(I suppose the in the non-interview settings, if this was a junior engineer, I'd might also add something like "And don't forget to make the pages you write be terminal-inspired, as the requirements say, see videos of people using [aerc], [mutt] and [himalaya]" - but I can see why this was not said in the interview. After all, testing that the candidate can read and comprehend a tiny requirements doc is a part of the interview)
In my interpretation the "righteous indignation" part is that the candidate tried to make sure that they are not ending up wasting their time and not chasing some pipe-dream (which is bad for both parties after all, leaves a sour taste in the candidate's mouth, and ... apparently does not further the good reputation of Kagi), yet ... that is exactly what ended up happening, because the fucking ~~money lenders in the temple~~ hiring mangers in the corposphere are not just useless individually, but are actively harmful as a cohort.
(Of course, the people deciding to hire hiring managers usually have good reasons to do so. And companies are not expected to babysit every candidate, but since almost always the power dynamics favors them they ought to be the more generous and proactive in the transaction. But at this point in the analysis we are just a few steps from declaring that it's cough late-stage capitalism's cough fault.)
