- The Exchange Online Protection outgoing servers (also known as "outbound servers") are listed in the Backscatterer.org reputation block list. (This listing can't be prevented because of certain kinds of spoofing attacks that can be directed against the Exchange Online Protection service and users.)
- The recipient email environment implemented the Backscatterer.org service in reject mode instead of in safe mode.
- The recipient email environment didn't add the Exchange Online Protection outgoing edge server IP addresses to the list of enabled mail senders.
- Use the Message Trace feature in the Exchange Online Protection Administration Center to determine the host name of the outgoing edge server that sent the users' mail items. For more information about how to run a message trace, see Trace an Email Message.
- Use the test that's provided by the BackScatterer.org service to determine whether the IP address is listed as the source of spam.
To resolve this issue, try one of the following methods:
- Contact the recipient mail administrator to have the specific Exchange Online Protection outgoing server IP addresses added to an enabled list to bypass the Backscatterer.org checks. For an updated list of Exchange Online Protection IP addresses, see Exchange Online Protection IP addresses.
- Contact the recipient mail administrator to recommend that he or she implement the Backscatterer.org service in safe mode, as recommended at the following Backscatterer.org website:
The Backscatterer DNSBL is a list of IP addresses that send backscatter. It's not a spammer list. The instructions on the Backscatterer website recommend that you not set up or use the reject mode for all incoming mail from the service. You should use the service in safe mode to block messages in which MAIL FROM resembles the following: