Microsoft figures that in five years time half the Exchange mailboxes in the world – perhaps 160 million mailboxes – will be running on its servers in its cloud infrastructure on Exchange Online.

At least that’s Microsoft senior vice-president Chris Capossela told Reuters going into the Reuters Global Technology, Media and Telecoms Summit the other day.

That’s one of the reasons Microsoft has been building massive data centers like Google – adding a reported 10,000 servers to the population a month – to host Exchange Online and its friends, Office SharePoint Online and Office Communications Online. Exchange Online is of course the SaaS side of Microsoft’s traditional Exchange mail and messaging software.

Microsoft will be charging a larger monthly fee for Exchange Online than it would to simply buy the traditional software but Microsoft’s margins may not be as good, Capossela said, since it has all that hardware overhead. It will all depend on how efficiently it can run a cloud. The customer of course saves himself CAPEX and OPEX charges by offloading them onto Microsoft.

Coke is already an Exchange Online customer having signed up for 70,000 seats and forsaking IBM’s Lotus Domino system. Energizer is another. Capossela apparently thinks Microsoft could run off any number of Lotus accounts.

Exchange Online runs on Exchange Server 2007 and comes in a standard configuration and – for customers with more than 5,000 employees – a personal, dedicated configuration, both with a guaranteed 99.9% uptime service level agreement.

Services include e-mail, shared calendaring and contacts – the specs say it supports Firefox 2.0+ – and with Active Directory there’s synchronization.

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