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Sender Policy Framework (SPF) is an email authentication method which ensures the sending mail server is authorized to originate mail from the email sender's domain. [1] [2] This authentication only applies to the email sender listed in the "envelope from" field during the initial SMTP connection.
DomainKeys Identified Mail (DKIM) is an email authentication method designed to detect forged sender addresses in email (email spoofing), a technique often used in phishing and email spam. DKIM allows the receiver to check that an email that claimed to have come from a specific domain was indeed authorized by the owner of that domain. [1]
An Author Domain Signature is a valid DKIM signature in which the domain name of the DKIM signing entity, i.e., the d tag in the DKIM-Signature header field, is the same as the domain name in the author address. This binding recognizes a higher value for author domain signatures than other valid signatures that may happen to be found in a message.
The leftmost result columns, labelled SPF and DKIM show DMARC-wise results, either pass or fail, taking alignment into account. The rightmost ones, with similar labels, show the name of the domain which claims to participate in the sending of the message and (in parentheses) the authentication status of that claim according to the original ...
The SPF verifier queries the Domain Name System (DNS) for a matching SPF record, which if it exists will specify the IP addresses authorized by that domain's administrator. The result can be "pass", "fail", or some intermediate result - and systems will generally take this into account in their anti-spam filtering.
Even if the SPF and DKIM validation fail, the receiving service can choose to validate the ARC chain. If it indicates that the original message passed the SPF and DKIM checks, and the only modifications were made by intermediaries trusted by the receiving service, the receiving service may choose to accept the email.
SRS is a form of variable envelope return path (VERP) inasmuch as it encodes the original envelope sender in the local part of the rewritten address. [2] Consider example.com forwarding a message originally destined to bob@example.com to his new address <bob@example.net>:
Also, if systems like SPF and DKIM became common, forged sender addresses would be recognized by these systems before reaching a C/R system. In some cases, C/R systems can be tricked into becoming spam relays. To be useful, some part of the message under challenge is generally included in the challenge message.