Hi
It is probably because the Zend email validation library is not uptodate with all the new generic Top Level Domain extension (gTLDs) that are being released.
Copy
/lib/Zend/Validate/Hostname.php
to
/app/code/local/Zend/Validate/Hostname.php
Navigate to line 539 and comment out the following lines of code. This is where the domain name is checked against an array of validTlds.
if (!in_array($this->_tld, $this->_validTlds)) {
// $this->_error(self::UNKNOWN_TLD);
// $status = false;
// break;
}