When you register a customer, the system asks you for an email which is normally associated to the company, if you need to send documents to different people associated to that company you must access Catalog > Customers. Customer tab Contact persons.
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 customer's main email will appear.
If you click on the email address field the system will show the different contact persons associated to choose which one you want to add in the sending. .