Z-Push (presumably Z is for Zarafa) is a FOSS implementation of the Microsoft Exchange ActiveSync protocol which is used to synchronize email, personal contacts and other items between a central server and a mobile device.

[2] Z-Push enables any PHP and non-PHP based groupware package to become fully sync-able with any ActiveSync compliant device with an appropriate backend.

[3] There is also a 3rd party project that implements a Zimbra Backend, allowing Z-Push to be used with a ZCS server (Including opensource edition).

This is as opposed to IMAP or SMTP which is a two way handshaking TCP protocol which can be both quite slow and costly in terms of battery consumption across high latency connections.

This means if no mail arrives there is no traffic on the TCP channel and the phone radios remain in low power receive only mode.