Custom headers
Message filters don't know about every type of message header. For example, any header beginning with a X such as X-Spam-score:. If you want to use a unknown header select "Customize" at the bottom of the leftmost list box (while editing the filter rule). Then either select a custom header from the list box or add one by entering its name in "New message header:". Omit the ':'. For example, if you have a Message-ID: <mailman.73.1328558416.25839.tb-enterprise@mozilla.org> header and want to test Message-ID: headers you would enter Message-ID (omit the trailing ':').
This creates a custom header that is stored using the mailnews.customHeaders setting. You can use it with other filters. It will show up in the normal list of headers, you don't need to use Customize again.
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