If they are issuing a resend on the message, it might still have the “next step” recorded on the message. Try sending a brand new message.
Check and see what nslookup returns on the user PC as well. I don’t think it pulls from there at all, but worth checking.
Check the source on the returned message. While your ns might return mx1.jen.com.cn , some other server in the path might be forwarding that to mx.badservername.com . It might not be your ns that is wrong, but one further along the trail.
If that doesn’t help, I might be able to explain the difference in DNS nslookup and what domino sees. I notice your running Windows. It has a DNS cache service that, from experience, isn’t queried by nslookup but is used by other API calls. So if this DNS cache becomes corrupted with invalid values then some applications might behave badly.
Stop the router service, then try restarting the Windows “DNS Client” service. Next start the router service and see if that improves things