If you read the assignment, you can see that the majority of tasks is about UI and user-facing features ("Should have basic email viewing + sending functionality", "Does not have to handle rich text messages, just plaintext", "take inspiration from existing terminal email tools like aerc, mutt, or even something like himalaya.", "It should feel fast and intuitive")
The assignment had exactly 1 line about actual back-endy stuff: "Can use a fake backend (DB, in-memory, etc) or real ..."
Now, what did your email asked for? There was a whole bunch of things about back-endy stuff, and exactly 1 (one) sentence about the user-visible things, the thing they cared about the most:
"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 and two accounts"
So maybe you intended this to be a complete design document, but it actually was not. This is because it did not actually did contain any parts the interviewer cared about. And that's one of the reasons why you got no feedback - you could do _anything_ for the backend and they would likely still accepted that, they simply did not care if you used Pulumi or terraform or "start.sh" or "docker compose up"
And no, you don't need to guess the grading, you just need to read the assignment carefully. What do they seem to want? What kinds of things do they mention a lot? What kind of things do they mention in passing?