Email tips.
DMARC is an open email Internet standard detailed in RFC 7489.
Domain-based Message Authentication, Reporting, and Conformance (DMARC) works with Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) to authenticate mail senders and ensure that destination email systems trust messages sent from your domain. Implementing DMARC with SPF and DKIM provides additional protection against spoofing and phishing email. DMARC helps receiving mail systems determine what to do with messages sent from your domain that fail SPF or DKIM checks.
Sample DNS TXT record DMARC setup:
dig txt _dmarc.mydomain.com +short
"v=DMARC1; p=reject; pct=100; rua=mailto:dmarc@mydomain.com"
Microsoft has a good write-up here https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/use-dmarc-to-validate-email?view=o365-worldwide
Prajit Sindhkar also has another good write-up here https://medium.com/techiepedia/how-to-report-dmarc-vulnerabilities-efficiently-to-earn-bounties-easily-f7a65ecdd20b
ProtonMail has yet another good post here https://protonmail.com/support/knowledge-base/anti-spoofing/