> SMTP is therefore vulnerable to man-in-the-middle attacks. Man-in-the-middle is an attack where communication between two servers is intercepted and possibly

> SMTP is therefore vulnerable to man-in-the-middle attacks. Man-in-the-middle is an attack where communication between two servers is intercepted and possibly changed without detection. While it's true that MTA-STS can protect email in transit from downgrade attacks, it is not intended to prevent changes to email content. In fact, even with perfect MTA-STS adoption it still won't prevent the MTA itself from changing the message contents (the message passes through every MTA unencrypted). If you want to protect the authenticity of your email, use DKIM. If you want to prevent MTA's from stripping the DKIM header, use DMARC. Google also uses ARC to sign messages, but that's not end-to-end like DKIM is.