How to uniquely identify a message a contextual gadget?

I need uniquely identify a GMail message from my gadget because I need to download the message (planning to do it using IMAP). Reading the developer's guide I guess that my best choices are extracting the message by using Gmail-MessageID extractor. This is my extractor definition in the manifest:

`<Extension id="messageid" type="contextExtractor">
    <Name>Message ID Extractor</Name>
    <Url>google.com:MessageIDExtractor</Url>
    <!-- <Param name="message_id" value=".*"/> -->
    <Triggers ref="CtxSaveMsgGadget"/>
    <Scope ref="messageidScope"/>
    <Container name="mail"/>
</Extension>`

The filter is commented because I'm testing why this extractor do not triggers the gadget, no matters if the filter is enabled or not, the gadget never shows. My questions are: 1. I'm in the good path to universality identify the message? Keep in mind that I'm planning to get the message using IMAP 2. In any case: sho开发者_开发技巧uld'n that extractor triggers the gadget in every message? What is wrong with the extractor? If I use the body extractor every thing works fine.

Thks in advance