When a donor creates a recurring donation using Soapbox Engage's Donations app, future payments are processed directly by the payment gateway selected for the Soapbox Engage account. When those future payments are processed, the Donations app records the transaction, but at this time does not send a notification email to the donor.
The most common approach to informing donors about successful future recurring payments is via the CRM database, such as a Flow in Salesforce, or an automation in Microsoft Dynamics.