The Twilio API will return this message if you attempt to send an SMS or MMS to a recipient who has previously replied STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, or QUIT to your Twilio number. You will not be charged for the message attempt.
To be able to send messages to such recipients, they must text START or YES to the Twilio number to opt in again.
Note that when a user opts out of a phone number that belongs to a Messaging Service, the user is also opted out of receiving all messages sent from that particular Messaging Service.
If you have further questions about SMS STOP filtering, see the following articles: