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When the indexing process fails for any reason, you can now restart the process from the right context menu of the file. (Build 1.5.6 and above)
Restarting the index button is available only for PST, OST, MBOX, and OLM files.
For "Directories" the added or removed files will be automatically index and the count will be reset. (build 2.4.2 amd above)