X-msfbl Review

When a recipient clicks "Report Junk" or "Phishing" in Outlook, Microsoft generates a complaint. That complaint is sent back to the original sender via an FBL. If a sender accumulates too many complaints, their IP address or domain is added to the x-msfbl block list.

While not a standard HTTP header defined by the IETF (Internet Engineering Task Force), the x-msfbl header serves a diagnostic and filtering purpose. It is typically generated when Exchange Server uses the service (part of Exchange Online Protection or EOP) or when an on-premise Exchange server communicates with Microsoft’s cloud-based threat intelligence. x-msfbl

: The value of this header is typically a Base64-encoded string. It contains internal metadata such as the recipient’s local part, domain, binding group, and a custom user string. When a recipient clicks "Report Junk" or "Phishing"

The x-msfbl header is injected when the agent identifies the sending IP address as belonging to a known malicious or spam-sending block list. Essentially, x-msfbl is a marker that says: "This connection was blocked because the source IP appears on one of Microsoft’s proprietary block lists." While not a standard HTTP header defined by