Just playing around with setting up a new site from scratch here and I noticed that international email validation is not present in input type=email html forms nor is it in the filter_var php email validation function.

Not a single international email listed on Wikipedia's page [0] validates.

And in fact, a few of the non international ones do not validate either. Mainly the ones with double quotes in the username.

[0] https://en.wikipedia.org/wiki/International_email#Email_addresses