I usually search application logs using Windows Explorer and it works fine. But this time, I tried to search for string @domain and Search Companion pop up appeared with error message "The Indexing Service query cannot be completed successfully because the volumes you have specified are not indexed." I knew that there was nothing wrong with indexing because all other searches were working fine.

Google search found a page Unsolved Annoyances that said you will get that error message if you search for strings that start with either of following characthers: !#@