The email you received has two parts. If the books on your account are in the 'The following books on loan to you have been renewed successfully:' section they have been renewed for you. Each book is renewed automatically 40 times. If the book is in the section 'items will not be renewed if another library user has requested that item, and any titles in the following section have not been renewed for this reason. Please return it/them as soon as possible to avoid paying fines.' then you would need to return it.