I had a website not let me enter a proton.me email address, when I changed it to my custom.fyi address, it worked fine. They wanted a three letter TLD.

If I asked this question during an interview and the candidate gave me this list of assumptions, I would recommend the candidate. This is exactly what I would be looking for by asking a vague question, not if they memorized the answer to a bunch of riddles, but how they thought and what their line of thought was for troubleshooting the answer.