spf, dkim and dmarc records

DKIM - Domain Keys Identified Email

DNS TXT record returning an X509 certificate to verify e-mail source

selector._domainkey.example.com    TXT    v=DKIM1;k=rsa;p=MIIBIjANBgkqhki....etc

DMARC -  Domain Message Authentication, Reporting & Conformance

DNS TXT record defining a SPF reporting policy

_dmarc.example.com    TXT    v=DMARC1; p=none; pct=100; fo=1; rua=mailto:postmaster@example.com

SPF - Sender Policy Framework

DNS TXT record specifying authorised email sources

@    TXT    v=SPF1 +mx -all