General: Add additional tricky email addresses to test, with comments.

No behaviour changes are needed due to these tests, but the test code
should capture the reasons for rejecting some of these addresses, so this
commit adds tests and comments.

Props tusharbharti.

Author: arnt

tests/phpunit/tests/formatting/isEmail.php

@@ -34,6 +34,33 @@ public function test_returns_false_if_given_an_invalid_email_address() {
 			'bob@your mom',
 			'[email protected]',
 			'" "@b.c',
+			'h([email protected]', // bad comment
+			'hi@',
+			'hi@[email protected]', // double @
+			/* The next address is not deliverable as described,
+			 * SMTP servers should strip the (ab), so it is very
+			 * likely a source of confusion or a typo.
+			 * Best rejected.
+			 */
+			'(ab)[email protected]',
+			/* The next address is not globally deliverable,
+			 * so it may work with PHPMailer and break with
+			 * mail sending services. Best not allow users
+			 * to paint themselves into that corner. This also
+			 * avoids security problems like those that were
+			 * used to probe the local network around web
+			 * browsers and servers.
+			*/
+			'toto@to',
+			/* Several addresses are best rejected because
+			 * we don't want to allow sending to fe80::, 192.168
+			 * and other special addresses; that too might
+			 * be used to probe the Wordpress server's local
+			 * network.
+			 */
+			'to@[2001:db8::1]',
+			'to@[IPv6:2001:db8::1]',
+			'to@[192.168.1.1]',
 		);
 		foreach ( $data as $datum ) {
 			$this->assertFalse( is_email( $datum ), $datum );