What action should be taken if a third party is sending a message for the customer outside of the Marketing Cloud?

Integrating with the Marketing Cloud SOAP API is essential in scenarios where a third party is sending messages on behalf of a customer. This API facilitates the seamless exchange of data between the third-party platform and the Marketing Cloud, ensuring that customer data, message content, and list management can be effectively synchronized. By doing so, the third party can send messages while keeping the Marketing Cloud updated with relevant information.

This integration is key for maintaining a consistent customer experience and marketing strategy, as it allows for centralized tracking and management of customer interactions across different platforms. Furthermore, using the SOAP API helps to maintain data integrity and enables the Marketing Cloud to have a comprehensive view of all messaging efforts attributed to the customer.

The other options, while potentially relevant in different contexts, do not address the immediate need for integration to facilitate external messaging in conjunction with using the Marketing Cloud effectively. Hence, the focus on leveraging the SOAP API for integration stands as the most appropriate action.