... Shouldn't that be in the message headers instead of message body then? Now it seems all the posts to the list come as inline attachments with the "X-Archive: encrypt" replacing the main body of the message.

Not really. It can be in the header, but can also be effective if it's the first line of the message body: _http://en.wikipedia.org/wiki/X-No-Archive_ (http://en.wikipedia.org/wiki/X-No-Archive) "Some software systems also do not archive if the first line in the body of the message contains this text. This is useful for those users who cannot change the headers of messages they send out. If the X-No-Archive header is set to "No", or the header is absent, a Usenet archive will not recognise a prohibition on the message's archival." -paul