ARF
The Abuse Reporting Format of RFC 5965: a machine-readable email format that mailbox providers use to send spam complaints and other feedback to senders.
What it means
RFC 5965 (2010) defines an extensible format and MIME type for reporting feedback about received email, as a machine-readable replacement for the assorted formats in use before. A report is a multipart/report message with the report type feedback-report: a first part that a person can read, a machine-readable message/feedback-report part with fields such as the feedback type and source IP, and the original message or its headers.
RFC 6650 (2012) is the applicability statement: it describes how to use ARF for abuse reports and authentication failure reports. RFC 6449 names ARF as the standard format for complaint feedback loops, its main use in practice.
Senders should process ARF reports automatically and suppress the complaining recipient. Some providers redact that address, so a recipient identifier inside the original message helps match the report.
Example
Content-Type: multipart/report; report-type=feedback-report
Feedback-Type: abuse
User-Agent: ExampleFBL/1.0
Version: 1
Source-IP: 192.0.2.10Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes