[Exim] how to prevent mail loops if exim is relief MTA for a…

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Autor: Marc Haber
Datum:  
To: exim-users
Betreff: [Exim] how to prevent mail loops if exim is relief MTA for a broken sendmail
Hi!

given the following scenario. S is an old machine running sendmail.
Basically, nobody knows exactly what's going on on that machine since
the sendmail is severely hacked and the guy who configured that
sendmail has left the company. S is smarthost and MX for a number of
customer domains.

Right besides S, we have E, a new machine built by me, with exim 3.13
waiting to take over from S. Customers are being moved over from S to
E slowly.

Right now, a fiber has been cut and our main uplink is down. Thus, the
queue on S is getting huge since it can't deliver outgoing mail, and
incoming mail is only drippling in through the backup link. The
machine has crashed twice during the last three hours.

To take some load off S, we have configured S to use E as a smart
host. Thus, S can hand outgoing mail off to E quickly and is now only
loaded with incoming messages. This makes the situation a little more
bearable.

However, we have most customer domains configured like this:

domain MX 10 mail.customer.example.com
domain MX 20 S

I am now concerned about what would happen if
mail.customer.example.com goes down. Would this cause the exim on E to
fall back to the secondary MX S, thus causing a mail loop? Can I
configure E to refrain from falling back to S in case S is a secondary
MX?

To make things worse, there are a few addresses that are handled by S
locally and need to be delivered to S in any case. In these cases, S
is primary MX, and mail loops won't happen here since S will handle
these addresses locally.

Any hints will be appreciated.

Greetings
Marc

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