Mobile subscribers who reply with a "STOP" message to your Plivo number are considered opted-out. Any subsequent SMS to these phone numbers are blocked by Plivo. If you attempt to send a message to an opted-out customer, these messages will be marked failed with error code 200.
However, you may acknowledge the receipt of a "STOP" keywords from the customer to let them know that they have been opted-out. Plivo allows you to send outbound messages to opted-out customers with the following conditions:
- The acknowledgment SMS should be sent under five mins of receipt of "STOP" keyword from the subscriber.
- At most, one SMS is allowed to be sent to opted-out customers.