Final answer:
When a third party sends a message on behalf of a customer outside of the Salesforce Marketing Cloud, unsubscribes should be synchronized between the senders to maintain compliance with email marketing regulations and best practices.
Step-by-step explanation:
If a third party is sending a message for the customer outside of the Salesforce Marketing Cloud, it is essential to maintain synchronicity in the subscription status across different platforms. The recommended action is that unsubscribes should be synchronized between senders. This means that if a subscriber opts out using a third-party service, this preference must be communicated back to the Salesforce Marketing Cloud to ensure that the user is not contacted again in future email campaigns, complying with email marketing best practices and regulations.
Engagement metrics from the third party will not automatically pass into the Marketing Cloud; therefore, an integration may be necessary if these metrics are needed within Salesforce Marketing Cloud. Though integrating with the Marketing Cloud SOAP API could facilitate the syncing of engagement metrics, the immediate concern in the context provided is the synchronization of unsubscribe statuses between systems to prevent compliance issues.