The take home here has to be that, although there are a number of strong open source contenders for the Exchange Server crown, none is a true “plug in and go” replacement. To get the full groupware functionality an Outlook add-on of some description needs to be installed on every client and even then you don’t get a full 100 per cent replication of the functionality of an all-Microsoft set-up. Still, they’re all getting there and getting closer with every new release.
If pushed we’d have to say that the Z-twins (Zimbra and Zarafa) offer, arguably, the best level of Exchange/Outlook compatibility, with little to choose between them, and Scalix comes a close third. But then buyers are unlikely to base their decisions solely on the Outlook experience, with lots of other features and omissions to consider which could swing the balance in other directions.
Neither are the products mentioned here the only open source Exchange alternatives. Others worth researching include teamXchange from VIPcom – this makes use of server-side MAPI emulation from the OpenMapi.org project – plus Kolab and Horde. These too are all playing catch-up and, ultimately, it’s unlikely that any will ever topple Microsoft from the top of the messaging/groupware heap. But that doesn’t stop them trying, with worthwhile benefits to be had for those brave enough to give them a go.