Mobile subscribers who reply with a "STOP" message are considered opted out. Any subsequent SMS messages to these phone numbers are blocked. If you attempt to send messages 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 a customer to let them know that they've been opted out. System allows you to send outbound messages to opted-out customers with the following conditions:
- The acknowledgment SMS message should be sent within five minutes of receipt of the "STOP" keyword from the subscriber.
- At most, one SMS message is allowed to be sent to opted-out customers.