Index: wp-register.php
===================================================================
--- wp-register.php	(revision 3629)
+++ wp-register.php	(working copy)
@@ -13,7 +13,7 @@
 case 'register':
 
 	$user_login = sanitize_user( $_POST['user_login'] );
-	$user_email = $_POST['user_email'];
+	$user_email = apply_filters('user_registration_email', $_POST['user_email']);
 
 	$errors = array();
 
@@ -39,6 +39,8 @@
 	if ( email_exists( $user_email ) )
 		$errors['user_email'] = __('<strong>ERROR</strong>: This email is already registered, please choose another one.');
 
+	$errors = apply_filters('registration_errors', $errors);
+
 	if ( 0 == count($errors) ) {
 		$password = substr( md5( uniqid( microtime() ) ), 0, 7);