If you use Outlook online search and type a letter, same message appears “No matches found”
Solution is easy but really critic but there is another way which can help you to fix your problem and other problems. Rollup 6 for Exchange Server 2010 with SP1 has been announced. you can deploy Rollup 6 to your Exchange farm. here is the details: http://support.microsoft.com/?kbid=2608646
Anyway, you don’t want to deploy Rollup 6, here is the solution.
Be careful because playing registry can be dangerous.
After you install the update rollup, follow these steps to set the maximum number of query restriction nodes to 1,000,000 (decimal):