I've rearranged and divided up the Step 3 form (Mailing/Form/Uploade.php) a bit to improve usability. The following fixes are needed to make it more robust:
1. from_email and subject fields should be Required (and marked as such w/ asterisk)
2. Auto-responder (reply_id) field should be hidden by default - and then shown if Auto-respond to Replies? is checked. It is currently required - it should be conditionally required via formRule IF Auto-respond to Replies? is TRUE.