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The question title pretty much contains the question by itself, but as an example:

Instead of having an address like bob@example.com(where example is the sub-domain of the TLD com) could you have have an email address like sky@net, where the domain part consists just of net, which also happens to be a TLD?

Zavior
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1 Answers1

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It is technically possible for the owner of a TLD to add MX records in the TLD zone.

This would allow them to have email addresses like bob@example.

It's, however, not very common to do so and I can only imagine that there are plenty of cases where having such an email address can cause problems (email address validation that doesn't accept such addresses, etc.).

Here's an example of a TLD that currently does have MX records:

tt.                     86400   IN      MX      1 ASPMX.L.GOOGLE.COM.
tt.                     86400   IN      MX      10 ALT1.ASPMX.L.GOOGLE.COM.
pabouk - Ukraine stay strong
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Håkan Lindqvist
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    I believe there is an ICANN policy against this, but I've not been able to find it on their website. (I also have a vague memory of there having been an email address "ber@se", but that may be a legend I was told of when I started out in the early 90s...) – Jenny D Sep 12 '15 at 13:56
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    I know there is a prohibition against top-level MX records in the new gTLDs, because I personally wrote the code testing for that in the technical approval process. I suspect the requirement is buried somewhere in the vast amount of registration paperwork. It gets slightly more complicated by the addition of an MX record during the new TLD sunrise period (the "you used a name you thought would never be public" warning data), and of course old gTLDs and ccTLDs have much greater freedom to ignore ICANN's opinions. – Calle Dybedahl Sep 12 '15 at 14:16
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    Possible, but prohibited: [New gTLD Dotless Domain Names Prohibited](https://www.icann.org/news/announcement-2013-08-30-en) – Marcos Zolnowski Sep 12 '15 at 23:56
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    @sleske the edit seems a bit pointless, they still have MX records today. – Håkan Lindqvist Jun 22 '18 at 11:29
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    @HåkanLindqvist: Sorry, my mistake. I did a `dig` lookup on `a.root-servers.net`, which does not return an MX record. However, other server do return them. So it seems I don't understand the DNS :-(. Feel free to revert my edit. – sleske Jun 22 '18 at 11:59
  • E-mail address validators are _very_ often not complaint with the standard. – Trang Oul Sep 16 '21 at 05:37
  • Most clients and validators will accept a single domain label if it ends with a dot, which technically would be the correct notation anyway. In fact I used `nobody@invalid.` as a dummy address on several occasions and got away with it. – Bachsau Apr 08 '22 at 16:06