We had a fairly catastrophic issue this morning with our primary server (provider related).  I have a backup mx server setup that was working properly, queuing incoming messages. Suddenly it has started rejecting incoming email saying "No such user here".

 

The primary server is being brought online, but SmarterMail is not yet working there and in fact I have disabled the service. So I don't quite understand why the backup is now rejecting email. I can confirm that the rejected messages were routed to the backup server.

 

I have since taken the backup mx offline to stop further rejects, but I'm curious what would cause this to happen. Any thoughts?