SPF means sender Policy Framework
SPF is a protocol that aims to reduce spam. Within the SPF protocol, additional information is added to the DNS record of a domain: This record lists the mail servers that are allowed to send mail on behalf of that domain. Therefore, when a mail server is not in this field, but mail with the domain is sent as a sender, it is illegal and is seen as spam. SPF makes sure that there is less spam, but the real spammers simply use officially registered domains to get through the SPF-check.
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