Multiple Email Routing for the same domain

[alzuin]

Hello,
I successfuly used email routing/relay recipient to create a relay server for my customers with a internal mail server.
In some case, they have 2 Internet connections (a primary and a backup) with 2 different IP addresses, and a cisco router which is able to open the backup connection if the primary is down; in this situation the mail server is accessible from external using the backup IP Address and I have to manually modify the email routing destination address in ISPConfig.
I see a "sort by" field in email routing configuration: it seems like "priority" for DNS MX record: then I tried to configure a second routing server with the same domain but with different priority... but I recived an "domain_error_unique".
Now the question is: what is "sort by" field?
Can I configure a backup email routing server for a domain?
Thanks,
Alberto

[falko]

Domain: Type in the email domain or email address that you want to route to another server. You can also use an asterisk (*) as a wildcard. You can have just one routing rule per domain (ISPConfig will show you an error message if you try to add a second rule with the exact same domain), however if you use an asterisk there can be more than just one routing rule that applies to a domain.

Sort by: Postfix will process all routing rules from top to bottom and use the first one that applies and will stop then. If you have multiple routing rules that might match a certain situation, you can define the order with this field. A higher number means a higher priority, i.e., if you have two rules that apply, and the first has a priority of 8 and the second a priority of 5, then the first rule will be used by Postfix.