This alert checks when your a staff member is working on a customer account within AR Console and that customer does not have an AR contact set up properly (as shown below) then we send an email reminder so they can fix it. They should be marked as email preferred and include an email address. You may have a contact created but they also should be in the Accounts Receivable Contact box as well.
This allows invoices to be quickly and easily emailed from the AR console. ALSO the AR contact field is used by ID213 to email mini-statements to your past due customers to assist with timely collections.
(use ID766 to see all Contacts not associated with Customer)
The customer tab on the contact record also has to have the customer assigned there in order for this contact to be truly linked to the customer.