It depends how OutboxSMS has been configured. OutboxSMS can be configured several ways:-
1. No Delivery Receipt:
The email message will be returned with an error if it is not sent.
2. OutboxSMS Delivery Receipt:
OutboxSMS will send a report message if the message has been sent ok. It will also send the message back with an error if it fails.
3. SMS Delivery Receipt
Provided your mobile operator supports it, OutboxSMS sets a flag in the outgoing SMS to request a delivery report. When the message is received a delivery report SMS is sent back to OutboxSMS. OutboxSMS receives the message, converts it to email and emails it to the message originator.