What is Sender Policy Framework?

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What is SPF and how it is related to email authentication?

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Sender Policy Framework or SPF is a mechanism to prevent email spoofing attacks. The system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain comes from a host authorized by that domain’s administrators. The domain’s owner publishes SPF record as a TXT record in domain’s DNS, and receiving mail exchangers use that record to authenticate all emails coming from that exact domain.
For example the following SPF record for example.io domain:

 v=spf1 mx ip4:189.0.3.234/32 include:amazonses.com include:_spf.yandex.net -all

Informs receiving mail servers to check whether the IP of sending mail server is listed in example.io‘s MX DNS record or is equal to 189.0.3.234 or is an IP of Amazon SES service or an IP of Yandex Service. If nothing is matched, then receiving mail server will reject the message.

For more on SPF you could check the spec.

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