I don't think an @domain is required if it's an email address on the same mail server.

That certainly used to work on Gmail; I haven't tried recently.

(Obvious caveat being that any provider can trivially append @provid.er if you don't specify, so this isn't really proof that email is specified that way.)

Wow, that is literally the only reason I would want a TLD

ICANN forbids application records at the apex of a TLD. The only TLDs that do funky things like this are ccTLDs.

And if you try to actually use a bare TLD you will crash hard into collisions with unqualifed domains and search paths. So, fun as a hack to amuse fellow techies but not useful for anything real.

...is there someone out there who did this?

Not that I'm aware of. Although I wouldn't be surprised if google did it for their employees. They already own the google TLD, they could very easily make it point to the google.com emails.

How many HN-reading googlers have you just made to send 'Test' emails to themselves, I wonder!

Can you actually attach an MX record to a tld?

Huh, cool. Though I don't see a lot of those records myself. For example, `dig MX ai.` gives me the MX record for ai., but I can't see any MX records for as., bj., or dj.