X-msfbl

The "FBL" part of x-msfbl stands for . In email deliverability, a feedback loop is a system where an Internet Service Provider (ISP) – in this case, Microsoft (Outlook.com/Hotmail/Exchange) – sends complaint reports back to the sender.

As of mid-2026, x-msfbl in official articles (e.g., Microsoft Learn, Exchange Online headers list). It is considered an undocumented internal header , similar to X-MS-Exchange-Organization-* or X-Microsoft-Antispam headers. x-msfbl

In the complex world of web development, system administration, and email security, encountering obscure error codes or headers is common. One such string that often appears in server logs, proxy reports, and debugging tools is . The "FBL" part of x-msfbl stands for

X-Microsoft-Antispam: BCL:0; ... X-MS-Exchange-Organization-SCL: 0 X-MS-Exchange-Organization-AuthSource: ... x-msfbl: 1 Received: from mail.outlook.com ... x-msfbl in official articles (e.g.