"email with spaces"@gmail.com
"very.unusual.@.unusual.com"@mail.yahoo.com
"very.(),:;<>[]\".VERY.\"very@\\ \"very\".unusual"@strange.example.com
n@ai
Another server-side alternative is isemail.info, which does validate all of the above.
You can't actually register "email with spaces"@gmail.com or "very.unusual.@.unusual.com"@mail.yahoo.com with Google or Yahoo.
Both addresses pass pure syntax checks but then the validator kills it when it notices that Google or Yahoo won't let you register addresses like that.
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."