The badge number that you see on your app icon is a count of the conversations in Send that you haven't seen yet. It counts conversations, not messages. For example, if there is one conversation that you haven't seen in Send, but it has multiple messages, the badge count of your app will be one. If you have read the conversation in Outlook or another email app, the badge count will still show as one because you haven't seen the conversation yet in Send. Once you open the app, even if you haven't selected the conversation to read it, the badge count will reset to zero.