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An SPF record is a Sender Policy Framework record and is used to indicate to mail exchanges which hosts are authorized to send mail for a domain. It is defined in RFC 4408 and clarified by RFC 7208. From 2018 seems like this settings are required for most of mail servers.
SPF records are typically defined using the TXT record type. There is also an SPF record type, however it is deprecated and thus you should always have at least the TXT record definition present, even if you use the SPF type.
SPF records are defined as a single string of text. Here is an example record:
v=spf1 a mx ip4:18.104.22.168 include:_spf.google.com -all