Where does "Downstream server error" come from?

John R. Levine johnl at iecc.com
Mon Jan 20 00:11:34 UTC 2014

> Perhaps the host prior to the ones that had the error were doing recipient checking?

Nope, I got the error immediately after trying to connect, before it could 
even send EHLO.


> M.
> From: John Levine
> Sent: Sunday, January 19, 2014 17:56
> 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.

John Levine, johnl at iecc.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. http://jl.ly

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