There used to be someone who had the email address john@dk (maybe it wasnt john). Mx records on tlds are frowned upon but not technically invalid.

This is precisely the reason why you let the user bypass your check with a confirmation. It's likely that most of your users who enter something without a period made a mistake. If someone is using an address like 'john@dk', I think they'd expect a message saying 'your email is invalid'; and seeing a message that just says, 'are you sure?' with a 'yes' option would be perfectly acceptable.

Edit: sorry for the echo, bigiain - I saw your post right after adding the comment.

There is an MX record for ai, so the dot after the @ is strictly not required:

  ;; ANSWER SECTION:
  ai.			14400	IN	MX	10 mail.offshore.ai.

  ;; ADDITIONAL SECTION:
  mail.offshore.ai.	14400	IN	A	209.59.119.34

Yeah, good luck to whoever uses that address. I would expect many MTAs to have problems delivering to that, let alone web form validation.

Which MTAs do you suspect wouldn't deliver it? I've used internal zones of that form extensively without issue on all the common OSS MTAs (postfix, sendmail, exim).

I'm unaware of any common MTA software which wouldn't handle it.

Several of the hosted ESPs I used couldn't handle .aero and .mobi at the time they came out.

imperialWicket's approach would still work fine in that case when john@dk clicks the "Yes, that's really my email adress" button - and I suspect he'd be pleasantly surprised to see it work, I'll bet he's got a very high expectation of that address failing completely on most websites...

I have a cow-orker who's first name is "G" - he's very used to poorly-validating websites claiming that his firstname is "wrong".

Been a long time since I last saw "cow-orker". Or USENET, for that matter...

Given by his parents?

Yup.

I've got another friend/acquaintance who's got a fully legal (self chosen) single name. No first/middle/last name, just a "mononym". He has the expected hilarious outcomes with things like Google's "real name policy".

A prominent example of an interesting name is Caterina Fake.

He? So it's not Cher?

Nope. Can't even claim "acquaintance" whth Cher. Or Prince.

One of the guys that managed the Norwegian (no.) USENET hierarchy and used to be responsible for newsgroup creation for it used to go under the nickname Dr. No. Whether because of the no prefix on the hierarchy, or because people liked to complain about it whenever he rejected a proposed group, I don't know. But people repeatedly suggested campaigns to get the .no NIC to let him have the address dr@no. Never got anywhere though.

The dk zone still has an A record (as one of the few?), so http://dk./ is a working web site name (though it redirects the the longer official Danish NIC).

Doesn't work from here. Wonder if maybe the local DNS server isn't happy with it.

Chrome at least does not allow this. Curl may be a better test...

Works fine here in Chrome. Are you sure you're entering http://dk/ and not just dk? The latter will trigger a search instead.