For US/Canadian accounts sending through the registered messaging service, the opt-out keywords registered with your campaign (STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, QUIT) are handled by the carrier-level service: the customer receives the registered auto-reply ("You have been unsubscribed from <Company> and will receive no more messages. Reply HELP for help or START to resubscribe...") and further messages from your service to that number are blocked; START, YES or UNSTOP re-subscribes and HELP/INFO returns the help text. Sprwt itself does not change the customer's Receive SMS? preference when a STOP arrives — the reply is stored like any other inbound text (Chat / History > Received) and one received-message charge applies. If you want your own records to reflect the opt-out, switch Receive SMS? off on the customer manually.