When a Sales Order is created and the "To Be Emailed" field is checked to send out an email to the customer with customer defaulted to have transactions emailed and also have the preference Customers Default to Email Transactions checked. When the Sales order is saved the To Be Emailed field blanks out. The reason why this happens is because the email has already been sent out. If you would like to send another email or make modifications to the Sales Order and send the email out again one can manually check the box. This is default NetSuite behavior.