What is Backscatter messages in Exchange Online Protection?
It’s the automated bounce messages which is sent by mail servers usually due to incoming spam.
Because Exchange Online Protection is a spam filtering service , messages sent to suspicious and non-existing recipients are rejected. When this process happens Exchange Online Protection generates a NDR ( non-delivery report) message 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 Backscatter DNS Block List (DNSBL). The Backscatter DNSBL is a list of IP addresses that send backscatter messages. It isn’t a spammer list, and the Exchange Online Protection team does not remove the servers from the Backscatter DNSBL.