SPF record
"v=spf1" - used version SPF. - "+" - accept correspondence (Pass). This parameter is set by default. That is, if no parameters are set, then it is "Pass"; - "-" - Reject (Fail); - "~" - Soft reject (SoftFail). The letter will be accepted, but will be marked as SPAM; - "?" - neutral attitude; - "mx" - includes all server addresses specified in the domain's MX records; - "ip4" -the option allows you to specify a specific IP address or network of addresses; - "a" - we specify the behavior in case of receiving a letter from a specific domain; - "include" - includes hosts allowed by the SPF record of the specified domain; - "all" - all other servers not listed in the SPF record.