Specifying In-Reply-To, Message-ID and References headers in

When setting up headers for a mail message, I have a few
@headers[“X-foo”] = ‘blah’ lines, and all the X-whatever headers work
fine. However, I also need to set the References, In-Reply-To and
Message-ID headers, and for some reason, when I set those headers, they
don’t show up in the transmitted email. Is there something I need to do
to allow me to set them? I’m setting them to RFC2822-compliant values.