Introduction: The Plain Text Weakness
Historically, email is an incredibly insecure protocol. If Server A (Google) tries to send an email to Server B (a small company), and Server B doesn't support encryption, the email is sent in plain, readable text over the open internet. Hackers use 'Downgrade Attacks' to trick routers into stripping encryption so they can read these emails. MTA-STS fixes this.
The Strict Policy
MTA-STS (Mail Transfer Agent Strict Transport Security) acts like HTTPS for email routing. You publish a policy on your domain that says: "I exclusively support TLS 1.2+ encryption. If you cannot establish a fully encrypted tunnel to my IP address, do not send the email at all. Drop it."
Conclusion
MTA-STS is a vital upgrade that forces the aging email backbone into the modern, heavily encrypted era. Verify your MTA-STS records here.