It looks like Microsoft's proposed Yahoo acquisition is already attracting its share of buzzards: Open-Xchange, a prominent open source competitor to Microsoft Exchange, is looking to lure customers of Yahoo subsidiary Zimbra with an offer of steep discounts.

Zimbra, which released a major new version earlier this month - its first since being acquired by Yahoo - is seen by many as a direct competitor to Exchange, which wouldn't bode well for its chances if the Yahoo acquisition goes through.

Open-Xchange moved to take advantage of the situation on 25 February with a deal that will give Zimbra users a discounted two-year subscription to an equivalent Open-Xchange product.

Zimbra users need to supply their last two years' Zimbra groupware invoice, via a reseller or direct via fax, and Open-Xchange will offer a subscription for 50 percent of the price listed on the invoice.

"This is a disturbing time for many Zimbra users," said Open-Xchange chief executive Rafael Laguna, in a statement.

Laguna pointed out that the company introduced a Community Edition earlier this month, committing to release all server components, installation and administration tools under the open source GNU General Public License (GPL). The move is intended as further insulation against disruption to customers, no matter what becomes of the software maker. The standard Open-Xchange Server Edition starts at $898 (£457).

Zimbra, ironically, has made similar claims that its open source connections will protect customers' investments.

A free, open source version of Zimbra Collaboration Suite (ZCS) is available alongside the commercially supported Network Edition.

"We just made an irrevocable contribution of Zimbra 5.0 to open source," said Zimbra co-founder Satish Dharmaraj in a recent blog post to users. "No one can take that away. We will remain true and faithful to our contractual obligations and will do everything possible to make sure that our biggest assets - you - remain confident that we are here to stay and execute."

Zimbra released ZCS version 5.0 earlier this month, based on an Exchange-like server and an Ajax-based client that runs in a web browser. The new edition allows users to take the browser-based client offline, and also extends its reach to mobile devices such as the BlackBerry, J2ME-enabled handsets such as the Motorola RAZR and mobile web browsers such as that found in Apple's iPhone.

The new client includes numerous tweaks, including web-based instant messaging supporting multiple conversations and group chats and sharing inboxes and email folders with others, including a read-only option.

Like Exchange, ZCS supports the Outlook client, as well as Apple Mail, Eudora and other widely used applications. The new version offers native email, contacts, calendar and task synchronisation from Zimbra to Outlook 2007.

ZCS is based on open source technologies such as Postfix, MySQL, OpenLDAP and Apache, and the server is itself released under the Mozilla Public License (MPL). The web client is covered by the slightly different Zimbra Public License (ZPL), which requires an attribution to Zimbra.