Confirm where the NDN is coming back from by looking at the message headers. If it is coming back from the remote system, you will need to contact an IT staff person at that organization to troubleshoot this issue. Can you send to these addresses using gmail or yahoo?
If the NDN message is coming from your system, it could be a DNS lookup issue. In other words, your system is unable to resolve the alameda.k12.ca.us MX record.