Exchange 2010 SP1+ Search limitations
Have you ever wondered why your very large complex search query is failing to produce results? This could possibly be because your search query is too long.
A good rule of thumb is to stay under 256 characters when you write you complex search query and whenever possible, keep the query simple!
A good example of what not to do would be to create ten different search queries and concatenate the queries together into one long string.
If you are looking for the more complex queries, I would highly suggest moving to Exchange 2013 and SharePoint 2013 to get the full benefits of the FAST search capabilities.
Tony Geraghty, Senior Support Escalation Engineer, Enterprise Communications Services, Microsoft