Google has unveiled a service that lets businesses use Gmail as a back-up for Microsoft Exchange.

Known as Google Message Continuity, the service replicates all your Microsoft Exchange data on Google's servers. If your Exchange servers crash or you take them down for maintenance, your employees can open up a browser and switch to Gmail, Google Calendar, and Google Contacts. The service is based on technology from Postini, the business email outfit Google purchase in 2007.

"Since Gmail and Microsoft Exchange are constantly synchronized with each other," Google says, "you can seamlessly switch from one email environment to the other." Google uses synchronous replication and redundant data stores in an effort to ensure your data won't ever be lost.

Google also sees this as a tool that will ultimately help businesses make the switch from Exchange to Google Apps, the online suite that includes Gmail, Google Calendar, and Google Contacts. "Since Microsoft Exchange and Gmail are always in sync with one another, there’s no need to migrate email data when eventually deploying Google Apps. With Gmail, Calendar and Contacts available, people can get familiar with these cloud services without having to abruptly stop using their regular email system," Google says.

Mountain View is pulling out all the stops as it works to pry users away from Microsoft's businessware. In June of last year, Google rolled out a plug-in that lets you access Gmail from Microsoft Outlook, and this autumn, it introduced a similar tool for uploading Microsoft Office docs to its online word processor, Google Docs. The company is even suing government agencies that allegedly pick Microsoft Office tools without giving Google a fair shake.

Google Message Continuity is $25 per user per year for new customers or an additional $13 per user per year for existing Postini customers. ®