Index: units/users/users_event_handler.php
===================================================================
--- units/users/users_event_handler.php	(revision 15359)
+++ units/users/users_event_handler.php	(working copy)
@@ -755,6 +755,7 @@
 				return ;
 			}
 
+			$username_required = $object->isRequired('Username');
 			$this->RemoveRequiredFields($object);
 			$object->Load($user_email, 'Email');
 
@@ -786,8 +787,8 @@
 				$object->generatePassword();
 				$object->SetDBField('Email', $user_email);
 
-				if ( $object->isRequired('Username') )	{
-					$object->SetDBField('Username', $user_email);
+				if ( $username_required )	{
+					$object->SetDBField('Username', str_replace('@', '_at_', $user_email));
 				}
 
 				$object->SetDBField('Status', STATUS_ACTIVE); // make user subscriber Active by default