Hi @Greeshma_Sampath1 - last time I looked the External SMPP connector code was baked into the Campaign server code, so its not possible to alter the behaviour of the auto-responder directly. Its also difficult to tell when an auto-response has happened as there are no records to indicate a message was sent, other than the server SMPP logs.
You could try querying the MO responses in the inSMS schema yourself say every 15 or 30 minutes and finding matches that would have generated an auto-response and calling your API from there. Its not real-time, but depending on your legal jurisdiction, marketing opt-outs usually have a grace period before they are expected to be adhered to, so 15-30 minutes should be enough time to set the do not sms flag. You can also indicate that period in your auto-response SMS.
Cheers
Darren