On Tue, Apr 07, 2020 at 01:43:45PM -0500, Linda Pagillo via Exim-users wrote:
> Hi everyone! I posted something similar to the Postfix mailing list. I'm
> doing a bit of preliminary research. I have a few Windows-based mail
> servers. I would like to possibly set up Exim as a backup MX server for the
> Windows servers.
The feedback you got on the Postfix users list carries over largely
unchanged for Exim. The configuration file syntax is of course
different, but otherwise the key requirement for backup MX hosts is the
same for all MTAs:
* Don't accept on the backup MX what you would reject on the primary.
- This includes recipient address validation.
- This includes any IP reputation checks.
- This includes any content-based filtering.
The recipient validation requirement for backup MX hosts can be met
either by deploying an LDAP replica, or by periodically exporting
recipient list snapshots from LDAP as flat files to be indexed and
queried on the backup MX via some suitable local lookup mechanism.
Once your backup MX has policies that are sufficiently similar to the
primary MX, it is just a matter of relaying the mail to the primary when
it comes back up. Both Exim and Postfix will do that automatically when
configured as relay hosts rather than final destinations for a domain.
With suitable MX records in DNS:
example.org. IN MX 0 primary.example.org. ; Gateway to the mailstore
example.org. IN MX 10 backup.example.org. ; Relay to the primary
and the backup MX configured to accept mail (but not deliver locally)
for "example.org" from "strangers", relaying to the best MX when it
comes back up is a standard feature of all correctly implemented MTAs,
Sendmail, Qmail, Exim, Postfix, ...
--
Viktor.
