When we register a customer, the system asks us for an email which is normally associated with the company, if we need to send invoices to different people associated with the company we have to access Catalog > Customers > Customer file > Contact people.
In this section we register the different contact persons with an associated email, this way when we make any sales document and click on "Send" we will see the main email of the customer. By default, the program will show the email of the customer's file, you can not set by default more than one email.
If we click on the address box, the system will show us the different email addresses associated to this customer, to choose which one we want to add in this shipment.