CiviCRM fails validation for IDN email addresses, owing to the regular expression check in HTML_QuickForm's /HTML/QuickForm/Rule/Email.php
Regular expression checks against IDN emails are pretty futile since any TLD authority may define additional UTF-8 characters for use in their space.
Converting addresses to punycode (idn_to_ascii()) before validation should work.
To reproduce, enter any valid IDN which is not strict ASCII in a CiviCRM contact email and press save. In .nz namespace the available characters include āēīōū as well as regular ASCII, so test@ēxāmplē.co.nz is a "valid" (but non-existent) email address within .nz, and http://www.taiuru.māori.nz/ is an example domain using a macron.