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

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".

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).