The only two that I know of are below. Both BSD licences. Neither are documented as valid for Jira 6. But you should find that the first one works for version 5, and from my limited tinkering with email handlers, I suspect it would be quite easy to recompile for v6.
the second link which is posted by @ Nic is the free version and old one, you have to upgrade it your jira version.
or you can develop your own plugin check here for document
and you get sample plugin here
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