The Newsletters moudule cannot be used without entering password and username. The module stores the settings in the database, not in the web.config and checks whether the MailingService could be connected to your account. If your host allows fake username and password you can give a try, although this could be concerned as not a good practice. The reason that the newsletters module does not work without username and password is preventing users for using common accounts. Also the MailingService and SendMail method requre username and password as a string parametes.
All the best,
the Telerik team