The X-MSFBL (Microsoft Feedback Loop) header is an encoded string inserted into the headers of outgoing emails. Its primary purpose is to help high-volume senders identify which specific campaign or recipient triggered a "Junk" or "Spam" report.
If your network is clean:
x-msfbl is a non-standard, custom HTTP header or email header primarily associated with , specifically related to Exchange Online Protection (EOP) and Outlook.com . It is believed to be an internal tracking or filtering flag used to indicate that a message was processed as part of a False Positive or Block List feedback loop. x-msfbl
Check your SMTP logs. Look for the rejected recipient address and the exact error code (e.g., 550 5.7.1 x-msfbl ). Note the sending IP address. The X-MSFBL (Microsoft Feedback Loop) header is an
At its core, this header is a tracking mechanism. It is stamped onto emails as they pass through Microsoft’s mail gates. Its primary purpose is to facilitate the "Junk Email Reporting" loop—a feedback mechanism that allows Microsoft to correlate a specific email with a user's action (specifically, marking it as spam or junk). It is believed to be an internal tracking
: By processing these headers, senders can automatically unsubscribe users who complain, helping to maintain a high sender reputation and ensure better inbox delivery. SparkPost Support Technical Context : It follows the standard for custom headers (which always begin with Log Processing