What section of what standard says a TLD can't have an email server on it? Is AAA not allowed to host an email server on `aaa`[1], and have the email `sales@aaa`?

[1]: And "aaa" is a valid TLD; see the full list: https://www.iana.org/domains/root/db ; now, perhaps it is required to at least have a second level domain, but that's what I'm asking: is an MX record invalid on a TLD?