As a server, Kerio MailServer can be installed on a number of platforms, including Windows, Mac OS X and Linux. It also supports integration with Active Directory or Apple's Open Directory as well as Pluggable Authentication Module access for Linux, allowing simplified user and e-mail account management. And it supports an internal account database.
Kerio's site includes detailed instructions for using the iPhone as a client device and the company provides a wide range of configuration information and explains how to move from other mail and groupware solutions. Licensing is cheaper than Exchange and includes a variety of options such as bundled anti-virus solutions. Since the licensing is primarily user-based, there are no additional fees for mobile device or iPhone support.
Zimbra Collaboration Suite is another full-featured e-mail and groupware solution. Like Kerio MailServer, Zimbra can be installed on a number of platforms and supports a range of clients. Zimbra is available in several product editions , ranging from a basic open-source solution that does not include support for syncing data with either desktop or mobile tools to an extremely well-provisioned professional edition.
In that professional edition, Zimbra supports integration with Outlook and any Mac OS X apps that implement Apple's iSync framework, including Mail, Address Book and iCal. Unlike Kerio, Zimbra uses connector tools installed on client machines to manage data syncing with the server. And platforms other than Windows and Mac OS X will have limited access to collaborative tools beyond basic POP/IMAP for e-mail and Zimbra's rich Web interface.
Zimbra implements mobile device support, including support for the iPhone, Windows Mobile, Symbian, Palm and other devices as an optional add-on. Zimbra Mobile , as the package is called, is available for a flat $500 additional fee beyond the standard licensing terms.
Zarafa and Z-Push
Like Zimbra, Zarafa is available in both open-source and commercial variations, although it you want Outlook support, you'll need one of the commercial variations. Zarafa offers support for e-mail, centralized contacts, calendar, task management and public folders. It's designed primarily to use Outlook as a desktop client or a rich Web interface for collaborative features, and the Web interface, supports POP/IMAP access.