As a third-party SMS app, Pulse SMS can only hand off messages to Android to be sent. Android will make all of the carrier connections and report the sending state back to the app. If the message fails, that is your carrier explicitly saying that it couldn't be delivered. You could try restarting the phone. Of course, if you try to send a message from your phone's stock/native messaging app and it fails as well, the issue is not on Pulse SMS' side.