[Exim] Message-ID header

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I have had trouble subscribing to this list because Outlook 2003 does not add a Message-ID to outgoing mail, and all my messages to the list have been bounced back.

I've spent all morning going through the documentation at exim.org, to no avail. Can someone please tell me the code I need to add to my mailserver's config to get Exim to add a Message-ID header?
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