Why am I getting these 5/5 or 400/400 mail delivery failure message?

Most the time these messages are caused by either a forwarder or an auto responder. if an email account is forwarding the message to Gmail, Yahoo, or Hotmail, these companies have certain settings in place that will block the domains sending capability to their servers and that will cause it to bounce back giving the 5/5 message. If there is an auto responder it may be that the auto responder is responding to an email address that doesn’t exist. Another possible reason could be the account may be exploited and be sending out spam messages using the sendmail function and causes the 5/5 message or the 400/400 message.