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Sender Policy Framework (SPF) is an email authentication method that 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.
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.
This gives the TXT resource record to be looked up as: brisbane._domainkey.example.net. Note that the selector and the domain name can be UTF-8 in internationalized email. [9] In that case the label must be encoded according to IDNA before lookup. The data returned from the query of this record is also a list of tag-value pairs.
DMARC operates by checking that the domain in the message's From: field (also called "RFC5322.From" [2]) is "aligned" with other authenticated domain names.If either SPF (specified using the aspf field) or DKIM (specified using the adkim field) alignment checks pass, then the DMARC alignment test passes.
Sender ID is an historic [1] anti-spoofing proposal from the former MARID IETF working group that tried to join Sender Policy Framework (SPF) and Caller ID. Sender ID is defined primarily in Experimental RFC 4406, [2] but there are additional parts in RFC 4405, [3] RFC 4407 [4] and RFC 4408.
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>:
A wildcard DNS record is a record in a DNS zone that will match requests for non-existent domain names. A wildcard DNS record is specified by using a * as the leftmost label (part) of a domain name, e.g. *.example.com. The exact rules for when a wildcard will match are specified in RFC 1034, but the rules are neither intuitive nor clearly ...