To set up a real-time, API-driven way to be alerted when transactional messages fail to send out of Marketing Cloud, Northern Trail Outfitters should have an external system to receive and confirm callback and subscriptions. Callbacks are notifications that Marketing Cloud sends to an external system when certain events occur, such as message failures or bounces. Subscriptions are requests that specify which events and messages an external system wants to receive callbacks for.References:https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-apis.meta/mc-apis/transactional-m