With more than 60% market share today and growing to 70% by the end of next year, according to Gartner Inc., Microsoft Exchange rules the e-mail, calendaring and collaboration. But another analyst firm, Yankee Group, predicted last year that 23% of Exchange customers may switch to open source alternatives by the end of 2008.

As analyst firms go, it's a good guess that Duncan Greatwood prefers the Yankee Group. He's the CEO of PostPath Inc. in Mountain View, Calif., a Linux-based collaboration software [1] suite with an open file store that makes it easier to manage than Exchange's closed internal database. More important, claims Greatwood, is that the PostPath Server software is fully compatible with Exchange. He says it uses the exact same protocols as Exchange, such as MAPI and NSPI, "and a dozen or so others," [2] so even other Exchange servers think they're dealing with a digital sibling. But, he boasts, PostPath took the inefficiencies out of Exchange so IT won't have to put restrictions on storage for individual mailboxes, earning the affection of end users; who also won't have to give up their Outlook clients, unless they want to. PostPath offers a browser-bases mail client as well.

According to Greatwood, PostPath offers high availability, with back up servers chiming in within a heartbeat so no work gets interrupted. It uses SATA drives, to keep costs down and recently has been certified to work with EMC backup and restore tools.

Today, PostPath is deployed on premise on x86 servers, starting at around $4,000 for 60 seats. But Greatwood says in the next six months expect to see a software-as-a-service version.