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 with the Import Wizard.

• Include the Salesforce Contact or Lead ID in your source report, list view. etc.
• When setting up your import wizard, map that record ID to Campaign monitor field "Salesforce ID" (we create this Campaign Monitor custom field automatically).

Now every Campaigh Monitor member will have their contact or lead ID saved in a custom field. When the primary sync runs, we will try to find a matching contact or lead by that ID first, and if we can't, we will match on email.

Note- your import wizard should only include the ID you want to map to Campaign Monitor. If both IDs are included in separate lines, we don't control which line of the report, and therefore which ID, will actually be mapped each time the import runs.

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.