When the sender's email address is not assigned to any User/ Queue in CRM, the email activity tracker does not display the User/ Queue the email is sent to, but displays [object Object] for that email activity.
Work Around: The User/ Queue needs to be created in CRM with the email address of the sender. For example, if the emails were sent from no-reply@domain.com, then a User / Queue should be created in CRM with this email address.