Users in an Exchange Online Protection environment receive NDRs when they send mail to a recipient environment that uses the China Anti-Spam Alliance service

Applies to: Exchange OnlineExchange Online Protection


Users in a Microsoft Exchange Online Protection environment receive a nondelivery report (NDR) when they send email messages to a recipient. This occurs when the recipient's messaging environment uses the China Anti-Spam Alliance (CASA) service for mail security.

The NDR may include a message that resembles the following, where is an IP address that's in the range of Outgoing FOPE IP addresses:
The IP address was rejected by the Realtime Block List provider


This issue occurs if an outgoing Exchange Online Protection IP address is added to the CASA China Black IP list (CBL).


Contact CASA because the cause of the issue is rooted in the CASA CBL, and the resolution must be directed at CASA. Office 365 doesn't support CASA services. The following guidance is provided as-is and without any warranty to resolve unexpected mail rejections from recipient environments that use the CASA CBL service as a block list.

To resolve the issue, try the following:
  • If the recipient messaging system uses the CASA CBL, contact the recipient mail administrator to have the following Exchange Online Protection outgoing edge server IP addresses added to an allow list to bypass the CASA CBL checks:


The CASA CBL is a common real-time block list (RBL). RBLs are a list of IP addresses published through the Domain Name System (DNS) either as a zone file that can be used by DNS server software or as a live DNS zone that can be queried in real time. RBLs are most frequently used to publish the addresses of computers or networks that are linked to spamming. Most mail server software can be set up to reject or flag messages that were sent from a site that is listed on one or more such lists. The term "block list" is sometimes interchanged with the term "blocklist."

Microsoft is committed to enabling customers to have a secure email environment that is both spam-free and virus-free. As part of that commitment, Exchange Online Protection takes many steps to make sure that mail that is filtered through our network doesn't contain unsolicited commercial messages.

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