Although the iPhone's documentation names Exchange specifically, Apple actually licensed and implemented Microsoft's ActiveSync protocol, which was developed for Exchange. Apple is not alone. In fact, several other mail and groupware server products, both commercial and open source, are available that implement ActiveSync and allow mobile clients, including the iPhone and desktop applications such as Outlook, to access the same push and over-the-air features available from Exchange.
Here's a rundown of the alternatives they suggest:
- Kerio MailServer
- Zarafa and Z-Push
- Communigate Pro
- Outsourced or hosted solutions