I work at a local high school. The School Department email is provided by a single Exchange 2010 server running on Server 2008 R2. In late November the database stopped content indexing, sort of. The system is "healthy" services are running. The content indexing is in a perpetual crawl but never rebuilds the index. The search index has been manually deleted and reset multiple times.
The problem directly correlates with the date of the installation of "Update Rollup 2 for Exchange Server 2010 Service Pack 3 (KB2866475)" . The latest attempted fix was to run the AD Schema Update for Service Pack 3 again. After that Exchange automatically reset content indexing due to the age of the files.
Still in perpetual crawl and index is never rebuilt.