Tracking logs - recipientstatus field 'WHALEBACK'

Hi All,

I am going through some tracking logs to see why a recipient is not receiving messages from a particular sender.

I am seeing this:

This is the message hitting the HT server which then hands it over to the MBX server.  However you can see that the recipientstatus is 'WHALEBACK' and it also seems the message never hits the MBX server.

What does WHALEBACK mean?  I can't find any info on it anywhere.  Does anyone know?

Thanks,

Chris