The Email Defaults window, with its three views, allows you to enter text that will become the default text for the subject and message for all of the email invoices, purchases and statements that you send:
For Sales and Purchases, using the Email Defaults window, you can choose to include the number of the transaction in the Subject.
The default entries that you make in the window can be changed for individual forms, if you want, in the Review Forms Before Delivery window for the type of form you're emailing.