To configure pfSense to send an email notification when a fallback WAN connection goes down, you will need to do the following:
Set up an email server or use an external email service such as Gmail.
Configure pfSense to use the email server or service by going to System > General Settings and specifying the server's hostname and port, as well as the credentials for an email account that pfSense can use to send the notification.
Create a script that checks the status of the fallback WAN connection and sends an email notification if it is down. You can use the ping command to check if the connection is available and the sendmail command to send the notification email.
Schedule the script to run at regular intervals using the pfSense Scheduler. To do this, go to System > Scheduler and add a new task. In the "Command" field, specify the path to the script and any necessary arguments. In the "Recurring" field, specify how often the script should be run.
Alternatively, you can use the pfSense Package Manager to install an add-on that provides email notification functionality, such as the "Notify" package. This can simplify the process of setting up email notifications and may offer additional features such as the ability to customize the notification messages.
It's important to note that this is just a general outline of the steps involved in configuring pfSense to send email notifications when a fallback WAN connection goes down. The specific details of the configuration will depend on your specific requirements and the email server or service that you are using.