If contact is chosen on the Sales quotation and record has primary email attached to it, when sales quotation is confirmed this email address from Contact is not coming to sales order header to the email field even if it should. It works when sales order is created directly and contact is chosen so same logic should also follow if order created through quotation because the email is already know. Now it is left empty, or wrong email from customer record is coming to the order (depends on setup) but this should not happen since Contact with email already chosen and should be seen as default in this case.