mails_viewer can do 1 and 2. The default usage will not send out emails at all. But if you want, you can. So it's preview and conditional send.
mail_view requires you to write additional code. The pro is that you can preview it lively without doing the action again. The con is that you have to write the scenario for preview.
mail_view requires you to write additional code. The pro is that you can preview it lively without doing the action again. The con is that you have to write the scenario for preview.