From the FastMail blog regarding Mail on iOS:
"Upon seeing that the IMAP server offers support for Apple's push mechanism, the app sends the server a list of folders that its interested in knowing about changes for, and a push token. Just as described above, when something changes the IMAP server sends a message through Apple's push service, which causes the Mail app to wake and make IMAP requests to get the changes."
https://blog.fastmail.com/2016/12/21...lk-about-push/
They also note about their own app that:
"...there's arguably a privacy concern in that we're sending fragments of mail contents through a third-party service (though you already have to trust Apple if you're using their hardware so it's perhaps not a concern)."
I think as they say above, if you opted in to Apple's system, and can always choose "Fetch" instead of "Push", then really it is up to the customer how they handle this.
As I said, it is a popular request, so we will look seriously at what we can do here for customers.