Allow internationalised email addresses

[chaals]

Fix #845

[edent]

For completeness, is it worth adding 你好@example.com?

Also, 屋企.香港 appears to be a real domain. Is there an example domain by the .中国 registry?

Based on https://www.hkirc.hk/content.jsp?&id=3&#!/6 there are some "reserved" domains which won't be assigned. 個人.香港