Filters come in handy when we want certain messages bypass the inbox, and automatically labeled, or both, but so far we could do it only on incoming mails. Now Gmail allows you create a new filter or update and apply it to older mails also. Google Operating System has a tutorial on how to do this
Screenshot:
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Note: This feature is not yet available all the users yet
Source: Google Operating System
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wrote, on July 15th, 2006
This is a great feature. After all, no one knows better how to implement query tools better than Google themselves.