The default value is [DEFAULT_REPLY_TO]. You can configure your Default reply-to-email by clicking on the "Gear-Icon" (on the top-right corner) on your account. Then access your "Default Settings", and here you can configure the reply-to-email that you can use on every campaign. Here is a shortcut: Default Settings.
You can also change the reply-to-email in Step 1 of your campaign creation process.