Index: wp-includes/vars.php
===================================================================
--- wp-includes/vars.php	(revision 13576)
+++ wp-includes/vars.php	(working copy)
@@ -82,5 +82,10 @@
  */
 $is_iis7 = (strpos($_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS/7.') !== false);
 
+/**
+ * Whether the server software is OS X
+ * @global bool $is_osx
+ */
+$is_osx = (strpos(php_uname( 's' ), 'Darwin') !== false);
 
 ?>
\ No newline at end of file
Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 13576)
+++ wp-includes/formatting.php	(working copy)
@@ -1477,6 +1477,13 @@
  * @return string|bool Either false or the valid email address.
  */
 function is_email( $email, $check_dns = false ) {
+	
+	// OS X server is wonky with checkdnsrr - specifically on registrations. Overrides $check_dns if true
+	global $is_osx;
+	
+	if( $is_osx )
+		$check_dns = false;
+	
 	// Test for the minimum length the email can be
 	if ( strlen( $email ) < 3 ) {
 		return apply_filters( 'is_email', false, $email, 'email_too_short' );