We have recently removed a large number of old Contacts from Marketing Cloud in order to reduce the billable contacts that we're paying for. We did this by using the Trash Can icon/utility in All Contacts. We did not delete the corresponding records in Salesforce (so the old Contacts exist in Salesforce, but not in Marketing Cloud).
We are syncing Contacts between Salesforce and Marketing Cloud using a Synchronized Data Source (via Marketing Cloud Connect). The synchronization is configured to sync "All Records with Email Address".
We have noticed that some of the old records that we deleted from Marketing Cloud have been re-added to Marketing Cloud (presumably by the Synchronized Data Source). Does anyone know the criteria that the Synchronized Data Source uses to sync records? For instance, if an old record has been remove from Marketing Cloud, will the sync bring it back into Marketing Cloud if it is modified in Salesforce?
Thanks for your help!