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.