Users have an option to create an Invoice (or other Transaction) and select Save & Email when they need to email the transaction to customers. The Transaction Email Template is stored in NetSuite and currently it cannot be customized. User might use an option to create a new Email template and send it via "Email" button on the Transaction from e.g. Invoice.
Follow the steps to customize Transaction Email Template for an Invoice Example:
1. On the existing Invoice record (Transactions > Sales > Create Invoices > Lists), go to the Communications subtab > Messages.
2. Click on the Email button.
3. On the Email Message window that pops up, go to the Message tab.
4. On the Template field, select New from the dropdown to create a new template.
5. On the Email Template window that pops up, assign a Name. Then under the Template tab, assign a Subject and then choose Text Editor and type the message on the message box.
6. Click Save. The Email template will be saved and will be available in the Template dropdown.
7. Now, go back to the Email Message window, choose the created template from the Template dropdown.
8. Then, still on the Email Message window, under the Attachments tab, check the Include Transaction checkbox.
9. Click Merge & Send. The email will be sent with created email template and the transaction (Invoice) as an attachment.
Note: Enhancement #119409: Setup > Company > Setup System Email Templates: Ability to customize the system generated email notifications when Transactions are attached to this email.