Messaging works on store and forward mechanism. If an SMS fails during submission when billing and routing is already done for the message, there is no way you can resend/retry on same gateway. A DLR to this effect is issued immediately. However you can configure a fail-over gateway to handle such failure.

It may be noted that billing is already done for such message, hence fail-over gateway cost should be taken in consideration. Over and above stop loss feature will also apply.