The NDR may include a message that resembles the following, where xxx.xxx.xxx.xxx is an IP address that's in the range of Outgoing FOPE IP addresses:
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:
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.
Article ID: 2845046 - Last Review: Dec 20, 2016 - Revision: 1