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Spotlight indexing is turned off by default. To enable this feature, select Preferences from the Postbox menu, then click on the General icon. Select the checkbox next to "Allow Spotlight to search messages." To try it out (without waiting for Postbox to automatically index your messages for Spotlight), select Indexing from the Tools menu, and then Index This Folder (or Index All Folders). Messages in Postbox will now be included in Spotlight searches.