Users in a Microsoft Exchange Online Protection environment receive a nondelivery report (NDR) when they send email messages to a recipient whose messaging environment uses the UCEPROTECT service for mail security.
This issue occurs if an outgoing Exchange Online Protection IP address is added to the UCEPROTECT block list.
The cause of the issue is in the UCEPROTECT block list, and the resolution must be directed to UCEPROTECT. Therefore, we recommend that you contact UCEPROTECT.
The following guidance is provided as-is and without any warranty to resolve unexpected mail rejections from recipient environments that use the UCEPROTECT service as a block list.
To resolve this issue, try the following. If the recipient messaging system uses the UCEPROTECT block list, 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 UCEPROTECT checks:
The UCEPROTECT block list is a common real-time block list. Real-time block lists are lists of IP addresses that are 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. Real-time block lists 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.
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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