Opt-out allows customers to unsubscribe from SMS messages. Honoring opt-outs is legally required and must be immediate. Standard opt-out keywords: - STOP - UNSUBSCRIBE - CANCEL - END - QUIT Opt-out requirements: - Honor immediately (within minutes) - Send confirmation message - No further marketing messages - Maintain suppression list - Allow re-opt-in if requested Best practices: - Include opt-out in every marketing message - Easy-to-remember keyword - Track opt-out rates as health metric