Penetration tester Sabri Haddouche has reintroduced the world to email source spoofing, bypassing spam filters and protections like Domain-based Message Authentication, Reporting and Conformance (DMARC), thereby posing a risk to anyone running a vulnerable and unpatched mail client.What he’s found is that more than 30 mail clients including Apple Mail, Thunderbird, various Windows clients, Yahoo! Mail, ProtonMail and more bungled their implementation of an ancient RFC, letting an attacker trick the software into displaying a spoofed from field, even though what the server sees is the real sender.That means if the server is configured to use DMARC, Sender Policy Framework(SPF) or Domain Keys Identified Mail (DKIM), it will treat a message as legit, even if it should be spam-binned.

Source: Mailsploit: It’s 2017, and you can spoof the ‘from’ in email to fool filters • The Register