How is Campaign Monitor data linked to Salesforce records?

Campaign Monitor for Salesforce creates subscriber list membership and email tracking records in Salesforce. These are then linked to a Salesforce contact or lead record. The linking process works by matching the email address in Campaign Monitor to the one found in the standard Salesforce contact or lead Email field. 

• If no match is found - the records we create are not linked.
• If duplicate email addresses are found, we match the most recently modified record.

FAQs

How to override this behavior.
Using our import wizard, you can include the Salesforce Contact or Lead ID. You can then map the contact or lead ID to a particular field called Salesforce ID, which we create for you.

If you are adding the record for the first time
If the records you add are not in Campaign Monitor, then the process above is enough to link using the Salesforce ID.

If both a matching contact and lead exist
If we find both a matching lead record and the subscriber list membership record, it will be linked to both. 

Lead conversion
If a lead record is converted to a contact, our subscriber list membership and email tracking record will be moved to the new contact record.

The records are already in Campaign Monitor.
If the records already exist in Campaign Monitor, the process above still applies. Use the import wizard to update/add the Salesforce ID record in Campaign Monitor. Then, to re-parent the records, you need to perform a full refresh. This process will detect the Salesforce ID and use it to match.

Here is the support article on the import wizard.