Where does "Downstream server error" come from?

Scott Howard scott at doc.net.au
Mon Jan 20 00:32:30 UTC 2014

I've come across this error (or something very similar to it) before.  I
can't remember the exact product, but it turned out to be a transparent
SMTP proxy somewhere in the path - possibly on a UTM firewall, but I could
be wrong about that part...

Not overly helpful I know, but might point you in the right direction...


On Sun, Jan 19, 2014 at 2:55 PM, John Levine <johnl at iecc.com> wrote:

> 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.

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