This is mainly a simple little redirector for <mail id> -> mail gateway. It's primarily used in kernel development and kernel change logs.
If you are seeing this, you are doing it wrong. The syntax for using this is:
http://lkml.kernel.org/r/<message-id> - for marc.info links http://lkml.kernel.org/g/<message-id> - for gmane.org links http://lkml.kernel.org/p/<message-id> - for patchwork links (with fallback to gmane.org)