Advanced Spam Filtering Option: NDR Backscatter Fix the issue To make sure that messages like this won't be marked as spam ...

Advanced Spam Filtering Option: NDR Backscatter

Fix the issue
To make sure that messages like this won't be marked as spam in the future, either turn off this Advanced Spam Filtering option or add the sender to the sender allow list for the affected group of users. If you're not getting messages from a specific sender, we recommend adding their address or domain to your organization's allow list. To add an address or domain to the allow list, do the following.
Go to the spam filter page (opens in new window).
Choose your spam filter, choose Edit, and do one of the following. 1. To add a specific sender to your organization's allow list, scroll down to the Sender allow list, choose Add, and add the sender's email address, and then save your changes. 2. To turn off the Advanced Spam Filtering Option, scroll down and cancel the selection. 
Wait 15 minutes before making sure that your change was successful.

Helpful Links

Advanced Spam Filtering Options