MessageLabs has this interesting offering - Email Continuity Service.
This service provides on-demand email failover system that you activate when an outage occurs, enabling email users to continue sending and receiving messages through Outlook, Lotus Notes, web browser, or BlackBerry devices – without interruption.
MessageLabs Email Continuity Service supports Microsoft Exchange and Lotus Domino mail servers and also offers Blackberry integration.
Interesting solution. But seriously, how many takers will there be?
The probability of activating this service is pretty low, unless your infrastructure has no high-availability and failover capabilities.