When your custom app uses a list of capability.notification via something like this:
input name: "notificationDevice", type: "capability.notification", title: "Send notification to these device(s):", multiple: true
and then does something like
notificationDevice?.each {
it.deviceNotification(notificationText)
}
and one of the notification devices is an instance of this device, then the text is sent directly as an email. No other notification app needed.
This is how the existing community drivers that provide this same function (like LGKv3 Sendmail and GCal Search) work.