They're only valid if you can own that email address and The only one that looks valid there is the last n@ai, since apparently ai actually has an mx record. T

They're only valid if you can own that email address and The only one that looks valid there is the last n@ai, since apparently ai actually has an mx record. The first two are legal by rfc but not allowed by the individual providers. The third is not legit by rfc 2606 because example.com is a reserved domain. If it was just a RFC validation then it should validate all of those (except maybe example.com). But they go beyond that: "Furthermore, the validator is ESP specific, so we can go way beyond valid syntax checks, bring in specific requirement for Gmail vs. Yahoo vs. Hotmail."