What does the error lookup of host "xx.xx.xx" failed in yyy router mean?

You configured a manualroute router to send the message to xx.xx.xx. When it tried to look up the IP address for that host, the lookup failed with a permanent error. As this is a manual routing, this is a considered to be a serious error which the postmaster needs to know about (maybe you have a typo in your file), and there is little point in keeping on trying. So it freezes the message.

1. Don't set up routes to non-existent hosts.

2. If you must set up routes to non-existent hosts, and don't want zing, set the host_find_failed option on the router to do something r than freeze.

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