Are there requirements on the relays to be able to receive notifications?

Because phone batteries are limited, the OS doesn’t require the app to keep running in the background. When the user isn’t actively using their phone, we have to rely on a notification server. The notification server watches the relays for the recipient’s address; if it detects a new message, it sends a system-level push through iOS APNs (Apple Push Notification service) or Android FCM (Firebase Cloud Messaging). The hard part is that, if users choose their own unique relays, the notification server would have to monitor a huge number of relays.