I encountered the following NullPointerException:
18/08/04 14:29:59 INFO James.Mailet : RemoteDelivery: Unknown gateway host: unknown host
18/08/04 14:29:59 INFO James.Mailet : RemoteDelivery: This could be a DNS server error or configuration error.
18/08/04 14:29:59 INFO James.Mailet : RemoteDelivery: Exception caught in RemoteDelivery.run()
This is caused by logging information about a HostAddress that happens to be null.
The HostAddress is null because I had a misconfiguration in my config.xml: a gateway server that could not be found.
I fixed the issue for myself by entering the correct host name.
Still, NullPointerExceptions are a bad sign.
Also, the message seems to disappear from the queue. I can't find it in the spool directory and there is no attempt of delivery after a restart.
In other words, I seem to observe that a message gets lost when this occurs.