I found this in the findmail FAQ. I wonder if it's common enough
to be included into mhonarcs TODO list :-)
4. As a list contributor, I do not want my messages archived.
Before you send your message, please place 'X-No-Archive: yes' in the
header of your email message ( see DejaNews). We will be looking for
this and not archive any messages with this header (we also support the
'Restrict: no-external-archive' header.)
Subject: I have this great new idea
I have this great new idea for...
Most mail programs allow you to add this header line simply by making
it the first line in the message portion. The message recipients will see
the header line in the header rather than as part of the message. Do not
put it in the "Subject:" header line, however, as this will not work.