Worry no more about typos or replies being sent to a customer by mistake. With "undo send" enabled for your account, you can bring those replies back and make your edits before sending for good. 

Turn on "Undo Send" under Settings > My Settings > Preferences

You can set your undo period between 5 - 30 seconds. Then when you reply to a conversation you'll notification appear in the bottom left corner of the screen. 

Click "Undo" to return your message to the reply editor to make your changes.