[mpercival531]

iOS Mail app does not support IMAP IDLE like any ordinary email clients.

It only supports a proprietary IMAP extension that uses Apple Push Notification Services (APNS) as a sideband channel for IMAP servers to signal the iOS Mail app.

Last I researched this (like… years ago), most IMAP-based email providers that are listed by the iOS Settings have implemented the extension, except for Gmail and Exchange. Fastmail then got on the train since 2015.

Not sure what is with the tweet targeting Fastmail specifically though.

[admdly]

Fastmail have been singled out as they appear to have been given special treatment in the form of an APNS topic ID. Other hosts have been using a reverse engineered endpoint to generate certificates, which has recently been closed.

There’s some discussion on the Apple developer forums - https://developer.apple.com/forums/thread/778671. The solution for the OP there seems to have been they also will get special treatment, but there remains no route for others to use to get the same.

[jeffbee]

Ironically, Google does use APNS to support its own Gmail app on iOS, and for many other reasons. Just not for IMAP.