When you register a customer, the system asks you for an email which is normally associated with the company, if you need to send documents to different people associated with the company you must access Catalog > Customers > Customer file > Contact persons.
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In this section you can register the different contact persons with an associated email, this way when you make any sales document and click on "Send" the main email of the customer appears, but if you click right next to it, the system shows the different associated contact persons to choose which one you want to add in that shipment.