t5-extend-email-checks.php

<?php # -*- coding: utf-8 -*-
/*
Plugin Name: T5 Extend Email Checks
Description: Overrides the results of the functions <code>is_email()</code> and <code>sanitize_email()</code>. Allows for example <code>me@localhost</code> or punycode encoded email adresses by using PHP’s internal filter function.
Version:     2012.08.29
Plugin URI:  http://toscho.de/?p=2195
Author:      Thomas Scholz
Author URI:  http://toscho.de
License:     MIT
*/
! defined( 'ABSPATH' ) and exit;

if ( ! function_exists( 't5_extend_email_checks' ) )
{
	add_filter( 'is_email',       't5_extend_email_checks', 10, 2 );
	add_filter( 'sanitize_email', 't5_extend_email_checks', 10, 2 );

	function t5_extend_email_checks( $result, $email )
	{
		return filter_var( $email, FILTER_VALIDATE_EMAIL );
	}
}

Note that apostrophes are escaped by WordPress, so they are still invalid.