SPF, DMARC, and DKIM

SPF (Sender Policy Framework) is an email authentication record, which is essentially used to define which specific servers are permitted to send mail on behalf of a domain. 
 https://docs.ukfast.co.uk/email/spf.html 
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 Adding an SPF record 
 useful SPF record generator tool 
 SPF records need to be added as a TXT DNS record. 
 example: 
 "v=spf1 mx a ip4:185.216.77.122"