Failure of hardware is a reality and no matter how many precautions you may take in terms of failovers, you can never claim 100% uptime. Mimecast did just that and is paying the price when it recently suffered an outage. The problem is not just that it marketed 100% uptime, but also legally bound itself in the SLAs. This is a disaster waiting to happen.
http://www.cloudtweaks.com/2013/05/mimecast-email-servers-go-down-100-uptime-sla/



