Answer
The built-in automatic email triggered by 'Send E-Mail To' can't have its subject or body customized directly, and there's no dedicated screen for configuring it — it fires whenever a report finishes printing successfully to any logical printer. Two practical ways around the resulting duplicate-email problem: (1) create a dedicated Logical Printer used specifically for this purpose, route only the cases where you want your custom event's email sent through that specific printer, and add a condition to the Custom Event so it only fires when that particular Logical Printer was used — this narrows down exactly when the custom email gets triggered; or (2), for more flexibility, use the Report Rule Engine's own 'Send Email' action instead of a Custom Event (requires the FNDRRE component) — it supports fully customizing the subject, body, and attachment name, and its trigger conditions can reference the actual data values on the report, giving much finer control than a Custom Event over exactly when and how the email gets sent.