If Email Address is used as the Subscriber Key instead of ContactID or LeadID, Salesforce cannot map tracking data back to the CRM records, because Marketing Cloud Connect requires a direct relationship to a Lead, Contact, or User ID.

If Individual Level Tracking is not enabled in the Marketing Cloud Connector settings, tracking information is not pushed back to individual Contact or Lead records.

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Exact Extracts from Salesforce Documentation:

Source: Salesforce Help – Marketing Cloud Connect Tracking

→ "Marketing Cloud Connect tracking requires that Subscriber Key matches ContactID, LeadID, or UserID. If Email Address is used instead, tracking cannot be attributed."

Source: Salesforce Help – Individual Email Results Tracking

→ "Individual Level Tracking must be selected to enable tracking data writeback at the individual record level."

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