X-NAS-* headers

[Keith Hackworth <ke...@rpemail.com>]

One of my customers received a blank message from Korea which spoofed an
invalid email address on his domain.  I was looking at the message and it
contained 3 distinct "X-NAS" message headers that I don't recognize:

X-NAS-Classification: 0
X-NAS-MessageID: 43604
X-NAS-Validation: {3342587F-BB8C-4C06-B6B4-9B637E4CDC44}

Does anyone have any idea what would add an X-NAS-* header?  I've googled
it and didn't really find anything too relevant, other than people
complaining about spam messages.  I would like to prevent these, and it'd
be great if I could just block any message containing X-NAS in the
headers, but I wanted to make sure some client or isp doesn't add them
automatically.

Thanks,
Keith