Re: [exim] Backup mail server

On 2011-10-09 at 20:43 +0200, Fabien LUCE wrote:
> What I have mainly understood is firstly that when a mail server is
> looking for an external domain, a dnslookup router is looking for the
> highest priority MX or SRV entry of my domain zone file... it can only
> look for an A entry if nothing else exists.

Roughly correct, although SRV records are _not_ used by default, and
what matters is the published DNS, not whatever is currently in your
file on disk (it matters if you break replication).

> To set à backup mail server, another MX entry with lower priority should
> be written in the zone file.

Correct.

> And now I can install a second exim on a friend's machine (Sb as Server
> B).
> The first configuration I have thought: I tell Sb to respond for mydomain.org (in the domainlist) and configure routers to send to mydomain.org (with a dnslookup or manual router). Thus, my mail will be sent to my "master" server in a time defined by the "retry" part of my configuration file.
> And nothing else, even not any local delivery.
>
> Do you think it should work?

That is how things used to be done, but I strongly advise against doing
that for any new deployment today.

You need to think about how you deal with spammers and what happens to
messages going to left-hand-sides that don't exist and whether or not
you would be generating "back-scatter"; backscatter is bounces to
fraudulently used addresses, because you accepted responsibility for a
faked message and sending bounces, instead of rejecting at SMTP time so
that responsibility remained with the sender.

-Phil