There is an old Sci-Fi movie where invaders from space take up residence in unsuspecting humans. As the humans become more and more dependent on the aliens to provide life support, they end up eventually being completely replaced. In this case, the alien is GMail, the host is Exchange server, and the technology used to colonlize the victim is Postini. Very clever.
With the launch of Google Message Continuity, an e-mail backup service, Google wants to get a toehold in companies that run Microsoft Exchange servers for e-mail by offering themselves up as a backup solution should something go wrong with those servers. The idea is that in the event of an Exchange outage, workers could sign into Google accounts and get their regular work e-mail as usual through Google’s interface.