When targeting Office365 hosted mailboxes, Microsoft assigns a Bulk Complaint Level to incoming messages originating from bulk mailers.
The BCL rating is being assigned the same way the Spam Confidence Level (SCL) through an appended X-header.
A high BCL level indicates the related message is likely to be spam.
To determine the BCL level, Microsoft uses both internal and third party sources.
Please find below the Bulk Complaint Level thresholds:
|0||The message isn’t from a bulk sender.|
|1, 2, 3||The message is from a bulk sender that generates few complaints.|
|4, 5, 6, 7||The message is from a bulk sender that generates a mixed number of complaints.|
|8, 9||The message is from a bulk sender that generates a high number of complaints.|
A Bulk Complaint Level under ‘4’ will get you to the Inbox while a higher Bulk Complaint Level is very likely to direct your messages to the junk folder.
If you have high Bulk Complaint Level for your internal communications originating from a subdomain of your corporate domain name, try to whitelist the related mail-from or the originating IP address. It is a normal behavior for a mail server handling communications for a specific domain name to consider as suspicious incoming communications related to that domain name but not being sent by itself or its related IP ranges.
For more insights regarding Microsoft filtering systems, you can also have a look at the Microsoft Spam Confidence Level blog post.