In [[Microsoft 365]] organizations with mailboxes in [[Exchange Online]] or standalone [[Exchange Online Protection (EOP)]] organizations without Exchange Online mailboxes, EOP assigns a **bulk complaint level (BCL)** value to inbound messages from bulk senders. The BCL value is added to the message in an X-header and is similar to the [[Spam Confidence Level (SCL)]] that's used to identify messages as spam. A higher BCL value indicates a bulk message is more likely to exhibit undesirable spam-like behavior. Microsoft uses both internal and third party sources to identify bulk mail and determine the appropriate BCL value. # Sources [Bulk complaint level values | Microsoft Learn](https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/anti-spam-bulk-complaint-level-bcl-about?view=o365-worldwide)