Local user %1 has sent a significant number of messages that have resulted in domain validation failures. There have been ...

Local user %1 has sent a significant number of messages that have resulted in domain validation failures. There have been %2 such failures in the last %3 minutes.There have been %4 errors in total. This can happen when they send messages to domains that can't be routed to, or to users in remote domains that don't exist.