One of the many checks that mail servers carry out is to query DNS whitelists (DNSWLs). That method is fully discussed in [RFC 5782
]. The DNS [RFC 1034
] lookup is based on the connecting client's IP address, IPv4 or IPv6, and returns zero or more A records. The latter are IPv4 IP addresses in the range 127.0.0.0/8. Depending on the query, TXT records with varying content can also be retrieved. Query examples are given in Appendix A
Since the IP address is known as soon as the connection is accepted, this check can occur very early in an SMTP transaction. Its result can be used to counterweight policies that typically occur at early stages too, such as the Sender Policy Framework (SPF) (the last paragraph of Appendix D.3
of RFC 7208
is also illustrated in Appendix A
). In addition, the result of a DNSWL lookup can be used at later stages; for example, a delivery agent can use it to learn the trustworthiness of a mail relay in order to estimate the spamminess of an email message. The latter possibility needs a place to collect query results for downstream use, which is precisely what the Authentication-Results header field aims to provide.
Results often contain additional data, encoded according to DNSWL-specific criteria. The method described in this document considers only whitelists -- one of the major branches described by [RFC 5782
]. There are also blacklists/blocklists (DNSBLs) and combined lists. Since they all have the same structure, the abbreviation DNSxL is used to mean any. The core procedures of a Mail Transfer Agent (MTA) tend to be quite general, leaving particular cases to be handled by add-on modules. In the case of combined lists, the boundary MTA (see [RFC 5598
]), which carries out the check and possibly stores the result, has to be able to discern at least the color of each entry, as that is required to make accept/reject decisions. This document provides for storing the result when the DNSxL record to be reported is a whitelisting one.
Data conveyed in A and TXT records can be stored as properties of the method. The meaning of such data varies widely at the mercy of the list operator; hence, the queried zone has to be stored as well. Mail site operators who configure their MTAs to query specific DNWSLs marry the policies of those lists, as, in effect, they become tantamount to local policies, albeit outsourced. Downstream agents who know DNSWL-specific encoding and understand the meaning of that data can use it to make delivery or display decisions. For example, a mail filter that detects heuristic evidence of a scam can counterweight such information with the trustworthiness score encoded in the A response so as to protect against false positives. Mail User Agents (MUAs) can display those results or use them to decide how to report abusive messages, if configured to do so.
This document describes a usage of TXT fields consistent with other authentication methods, namely to serve the domain name in the TXT record. That way, a downstream filter could also consider whether the sending agent is aligned with the author domain, with semantics similar to [RFC 7489
At the time of this writing, this method is implemented by Courier-MTA [Courier-MTA
]. An outline of the implementation is given in Appendix B