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 <jo...@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. > >
