I have access to the mail configurations on the primary and (by proxy) the
secondary server, both of these running Exim 4. I have no control over the
tertiary server at the ISP, though I have confirmed that it does forward
mail to me as expected. The problem occurs when my primary server goes out,
leaving the other two alive. At that point, the secondary server
(mail.friend.org) gets my mail and looks up the available MXes. Since
mail.domain.org is not available, but mail.isp.net is, mail.friend.org
immediately forwards it on to the tertiary server (mail.isp.net). This
server then sees that mail.domain.org is not available, but mail.friend.org
is, and as such forwards the mail there. At this point the cycle repeats,
causing a mail loop and, eventually, bounced mail messages.
My question, then, is this: How do I configure mail.friend.org to only
forward to mail.domain.org and never to mail.isp.net? Or, more generally,
how do I configure the server to send to MX servers *only* of higher
priority than itself?
I tried setting "hosts_treat_as_local" to include the domain name and then
the IP addresses of the tertiary server, but then the secondary server just
bounced mail back immediately instead of queueing it for later delivery.
I'm guessing that's since it saw no other available servers to send to?
