DeltaChat, when using a self-hosted Chatmail server, has a service that detects incoming mails for a user account, pulls a token ID out of their account info (f

DeltaChat, when using a self-hosted Chatmail server, has a service that detects incoming mails for a user account, pulls a token ID out of their account info (file in the maildir), and sends a request over HTTP to the push server run by the DeltaChat team. This sends it on through APNS/FCM, then scrubs the data from memory. The notification only wakes up the app. No encryption needed. The project runs on a shoestring budget and has no problem delivering 15 million+ push notifications per month without charging the users any money