Where does "Downstream server error" come from?

Sun Jan 19 23:06:18 UTC 2014

I have seen this error whenever the destination server responds with some strange response,  I have mostly seen it with iptables mucking packets.

Problem occurs on the transport layer not the application so the application has no idea what to make of it...

-------- Original message --------
From: John Levine
Date:01/19/2014 5:57 PM (GMT-05:00)
To: nanog at nanog.org
Subject: Where does "Downstream server error" come from?

I had some problems with incoming mail that I tracked down to a
configuration bug, two hosts on the same LAN configured to respond to
the IP address of the MX.  It's fixed now.

While it was broken, attempts to send mail on some other systems got
"421 Downstream server error."  That is not a message that any of my
mail software sends (I grepped for Downstream in the code, it's not
there) so I presume it's from some middle box.

Does anyone recognize the message, what produces it, and why?  There
was indeed stuff messed up downstream, but why turn it into a mystery
error message?

R's,
John

PS: I wonder how long it'll take for someone to suggest unhelpful
configuration changes on my host to fix the problem.