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... Scott On Sun, Jan 19, 2014 at 2:55 PM, John Levine <johnl@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.