[kuschku]

Well, wireshark + decompilation of existing software.

What I imagine, though, is a system similar to GCM, which would be not very helpful.

[vbezhenar]

I guess that this is achieved using a server-side communication: the Fastmail server send the notification to the Apple servers and the Apple server sends the notification to the user's device. So wireshark probably won't help.

[kuschku]

yeah, as I speculated, similar to GCM. Sadly, there isn’t an open standard for this.