Mobile subscribers who reply with "STOP" messages to your Plivo number are considered opted-out and any subsequent SMS to these phone numbers are blocked by Plivo. Such messages will be marked failed with error code 200.
However, You can acknowledge the receipt of "STOP" keywords from customers to let them know that they have been opted-out. To enable this Plivo allows you to send outbound messages to opted-out customers with the following conditions:
- The acknowledgement SMS should be sent under 5 mins of receipt of "STOP" keyword from subscriber.
- At most 1 SMS is allowed to be sent to opted-out customers.