This is the error I'm receiving when an external client tries to send us mail...
A non-delivery report with a status code of 5.0.0 was generated for recipient rfc822;email@example.com (Message-ID <firstname.lastname@example.org>).
Cause: This indicates a permanent failure. Possible causes : 1)No route is defined for a given address space. For example, an SMTP connector is configured, but this recipient address does not match the address spaces for which it routes mail. 2)Domain Name Server (DNS) returned an authoritative host not found for the domain. 3)The routing group does not have a connector defined û mail from one server in the routing group has no way to get to another routing group.
Solution: Verify that this error is not caused by a DNS lookup problem, and then check the address spaces configured on your STMP connectors. If you are delivering Internet mail through an SMTP connector, consider adding an address space of type SMTP with value ô*ö (an asterisk) to one of the SMTP connectors to make routing possible. Verify all routing groups are connected to each other through a routing group connector or another connector.
Here is what the NDR says that the external clients receive...
Your message did not reach some or all of the intended recipients.
The email system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator. #5.0.0>