Index: modules/user/user.module
===================================================================
--- modules/user/user.module	(revision 3187)
+++ modules/user/user.module	(working copy)
@@ -2340,6 +2340,7 @@
 }
 
 function user_register_validate($form, &$form_state) {
+  $form_state['values']['mail'] = trim($form_state['values']['mail']);
   user_module_invoke('validate', $form_state['values'], $form_state['values'], 'account');
 }