The first Thing I need to ask is did you actually send the messages? These MTA failure messages often come back to genuine email accounts when the address has been used by spammers. In that scenario it is called backscatter. https://en.wikipedia.org/wiki/Backscatter_(email) If it is genuine mail you are sending, there is a very high probability that misconfiguration of the network, probably on the recipient end is the problem. Perhaps misconfigured port forwarding on a firewall. But without knowing exactly where you are sending to. It is difficult to be at all precise.