Sun Java System Communications Services software is highly reliable. Initially built for the service provider market, the software was designed from day one to meet the strict uptime requirements demanded by service providers and their millions of users. With more than 220 million mail boxes and 100 million calendar seats sold to service providers, enterprises, and other organizations, the Sun Java System Communications Services platform has been battle-tested in the real world, and has stood up to the test. Sun Java System Communications Services software integrates with Sun Cluster high-availability clustering software, to deliver virtually continual availability and rapid recovery, even if hardware failure does occur.
Microsoft Exchange cannot claim a similar story of reliability. Its complexity and typically distributed deployment often lead to system outages. Running multiple servers means an increased likelihood of downtime due to hardware problems. According to a March 2003 Radicati study [Messaging Total Cost of Ownership 2003 in Enterprise and Service Provider Environments, The Radicati Group, March 2003] , Microsoft Exchange 5.5 users experienced unscheduled downtime more than twice as frequently as Sun users. The resulting dollar impact is an estimated annual downtime cost per user of $132 for Microsoft Exchange. Sun had a downtime cost of just $53 per user.