SPF record.

SPF record is a textual information about real senders for a given domain. MTA receiving the message is able to determine the authenticity of the sender's source.

Sample SPF record is:
@ IN SPF "v=spf1 a mx ip4:1.2.3.4 ip4:4.3.2.96/30 ip6:2001:aaaa:b:abcd::/64 a:nazwa-fqdn-ip.tld -all"

Sample SPF rules for MTA Exim.

First check SPF in Exim:

server#exim -bV
Support for: crypteq IPv6 Perl OpenSSL move_frozen_messages Content_Scanning DKIM DNSSEC Event OCSP PIPE_CONNECT PRDR SPF DMARC TCP_Fast_Open Experimental_SRS

Next, we can check SPF:

acl_check_recipient:

    warn	set acl_m_slow	= 0
    warn	set acl_m_spfw	= 

    deny
        !authenticated	= *
        condition	= ${if !eq{$acl_m_spfw}{1}}
        spf		= fail
        message		= SPF: $sender_host_address is not allowed to send mail from $sender_address_domain: $spf_smtp_comment
        logwrite	= SPF: $sender_host_address is not allowed to send mail from $sender_address_domain: $spf_smtp_comment

    deny
        !authenticated	= *
        condition	= ${if !eq{$acl_m_spfw}{1}}
        spf		= permerror
        message		= SPF: $sender_host_address is not allowed to send mail from $sender_address_domain: $spf_smtp_comment
        logwrite	= SPF: DENY, $sender_host_address is not allowed to send mail from $sender_address_domain: $spf_smtp_comment
        set acl_m_slow	= 1

    deny
	!authenticated	= *
	condition	= ${if !eq{$acl_m_spfw}{1}}
         spf		= none
         !verify	= sender/callout=30s,random,postmaster
    	 message	= SPF: Sender not have SPF record and SPF info does\n	\
    	    	    	       not grant $sender_host_address explicit\n	\
                               authority to send mail from $sender_address_domain.
         logwrite	= SPF: DENY, We check sender and we block messages from $sender_address_domain because sender test is invalid: $spf_smtp_comment
         set acl_m_slow	= 1

    defer
        !authenticated	= *
        condition	= ${if !eq{$acl_m_spfw}{1}}
        spf		= softfail : neutral
        message		= SPF: We greylist messages from $sender_address_domain\nbecause the domain's reputation is questionable:\n$spf_smtp_comment
        logwrite	= SPF: DEFER, We greylist messages from $sender_address_domain because the domain's reputation is questionable: $spf_smtp_comment
        set acl_m_slow	= 1

    defer
	!authenticated	= *
	condition	= ${if !eq{$acl_m_spfw}{1}}
        spf		= temperror
        message		= SPF: $sender_host_address currently is not allowed to send mail from $sender_address_domain: $spf_smtp_comment
        logwrite	= SPF: DEFER, $sender_host_address currently is not allowed to send mail from $sender_address_domain: $spf_smtp_comment
        set acl_m_slow	= 1

    warn
        !authenticated	= *
        spf		= pass
        add_header	= X-SPF: Result=$spf_result , comment=$spf_header_comment
        logwrite	= SPF: INFO, Result=$spf_result, comment=$spf_header_comment, sender=$sender_address_domain: $spf_smtp_comment
        set acl_m_slow	= 0

    warn
        !authenticated	= *
        spf		= none
        message		= SPF: $sender_address_domain\nreputation is questionable:\n$spf_smtp_comment.\n
        logwrite	= SPF: Result=$spf_result, comment=$spf_header_comment, sender=$sender_address_domain: $spf_smtp_comment
        add_header	= X-SPF: INFO, Result=$spf_result , comment=$spf_header_comment
        set acl_m_slow	= 2

    warn
        !authenticated	= *
        condition	= ${if match {${lookup dnsdb{txt=$sender_address_domain}{$value} } }{\\+all} }
        add_header	= X-SPF-WARN-ALL: $sender_address_domain allows +all in SPF policy
        logwrite	= SPF: WARN, $sender_address_domain allows +all in SPF policy
        set acl_m_slow	= 2

    warn
        condition	= ${if eq{$acl_m_slow}{1}}
        condition	= ${if !eq{acl_m_spfw}{1}}
        delay		= 10s

    warn
        condition	= ${if eq{$acl_m_slow}{2}}
        condition	= ${if !eq{acl_m_spfw}{1}}
        delay		= 60s

To fully check the sender, apply the simultaneous SPF, DKIM and DMARC policies.