It is not mandatory to associate a contact person with each customer. However, to send an email, you must associate at least one email address with the customer, and the same email must be selected under Email Communications when creating Recurring Invoices.
To verify whether the email address is selected under Email Communications:
Insight: Only future child invoices will be sent automatically if the Recurring Profile preference is set to Create, Push, and Send Invoices or Create, Charge, Push, and Send Invoices. For past child invoices, you can edit the invoice and update the Email Communications to send the email manually.
Books
Online accounting software
for small businesses.