I know this is possible, but my mind is swiss cheese at the moment and
I cant think how.
I want to add a header in a transport, ONLY if that header does not
already exist.
Actually, what I would really like to do, is:
1. Compare $sender_host_address with a list of networks.
If it matches, remove any X-Originating-IP header.
2. If there is no X-Originating-IP header (or if it was removed above),
then add one, with $sender_host_address as the value.
But I'm not sure this is possible.
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