Overview#Sender Policy Framework (SPF) is an email authentication protocol designed to detect forging sender addresses during the delivery of the email and is defined in RFC 7208
Sender Policy Framework alone though is limited only to detect a forged sender claimed in the envelope of the email which is used when the mail gets bounced. Only in combination with DMARC it can be used to detect forging of the visible sender in emails (email spoofing), a technique often used in phishing and email spam.
SPF allows the receiving mail server to check during email delivery that a email claiming to come from a specific domain is submitted by an IP Address authorized by that DNS Domain's administrators. The list of authorized sending hosts and IP addresses for a domain is published in the DNS Resource Records for that DNS Domain.
Sender Policy Framework Implementation#Compliance with SPF consists of three loosely related tasks:
- Publish a policy: Domains and hosts identify the machines authorized to send email on their behalf. They do this by adding additional records to their existing DNS information: every domain name or host that has an A record or MX record should have an SPF record specifying the policy if it is used either in an email address or as HELO/EHLO argument. Hosts which do not send mail should have an SPF record published which indicate such ("v=spf1 -all"). It is highly recommended to validate the SPF record using record testing tools such as those provided on the SPF Project webpage.
- Check and use SPF information: Receivers use ordinary DNS queries, which are typically cached to enhance performance. Receivers then interpret the SPF information as specified and act upon the result.
- Revise mail forwarding: Plain mail forwarding is not allowed by SPF. The alternatives are:
- Remailing (i.e., replacing the original sender with one belonging to the local domain)
- Refusing (i.e., answering 551 User not local; please try <email@example.com>)
- Whitelisting on the target server, so that it will not refuse a forwarded message
- Sender Rewriting Scheme, a more complicated mechanism that handles routing non-delivery notifications to the original sender
"v=spf1 ip4:192.0.2.0/24 ip4:198.51.100.123 a -all""v=" defines the version of SPF used. The following words provide mechanisms to use to determine if a domain is eligible to send mail. The "ip4" and "a" specify the systems permitted to send messages for the given domain. The "-all" at the end specifies that, if the previous mechanisms did not match, the message should be rejected.