Backscatter messages and EOP
Backscatter messages are the automated bounce messages that are sent by mail servers, typically as a result of incoming spam. Because Exchange Online Protection (EOP) is a spam filtering service, email messages sent to nonexistent recipients and to other suspicious destinations are rejected by our service. When this happens, EOP generates a non-delivery report (NDR) message and delivers it back to the "sender." Because spammers frequently use a forged or invalid "From" address in their messages, the sender address to which the NDR is sent may result in a backscatter message. When this happens, outgoing servers that are associated with the EOP network may be listed on the Backscatterer DNS Block List (DNSBL). The Backscatterer DNSBL is a list of IP addresses that send backscatter messages. It isn't a spammer list, and we don't try to remove our servers from the Backscatterer DNSBL.
According to the instructions on the Backscatterer website, the use of reject mode for all incoming mail isn't a recommended configuration or use of that service. It should be used in safe mode instead. For more information about implementing the correct backscatter configuration, visit the Backscatterer.org website.
For more information
See the "NDR backscatter" entry in Advanced spam filtering options