What Is MTA-STS? Email Encryption Your Business Needs

When someone sends your business an email, their mail server connects to yours to hand off the message. This connection can use TLS encryption to protect the email in transit -- similar to how HTTPS protects websites.

The problem is that TLS encryption for email is optional by default. A sending server will try to use encryption, but if the connection fails or is tampered with, it quietly falls back to sending the email unencrypted. The sender and receiver usually have no idea this happened.

This creates a real vulnerability. An attacker sitting between two mail servers -- known as a man-in-the-middle attack -- can strip away the encryption and read or alter emails without either party knowing. Sensitive business communications, invoices, contracts, and client data could all be exposed.

MTA-STS is an email security policy that tells sending mail servers: "Only deliver email to my domain over an encrypted TLS connection. If you cannot establish a secure connection, do not deliver the message at all."

It works through two parts that you publish on your domain:

You might already have SPF, DKIM, and DMARC configured for your domain. These protocols are essential -- they verify that emails are genuinely from your domain and have not been altered. But they do not protect emails while they are being delivered between servers.

MTA-STS fills a different gap. While SPF, DKIM, and DMARC protect the authenticity of your email, MTA-STS protects the privacy and integrity of email in transit. Together, they form a complete email security setup.

MTA-STS has a companion protocol called TLS-RPT (TLS Reporting). When you set up TLS-RPT, sending servers send you daily reports about whether they were able to establish encrypted connections with your mail servers.

These reports tell you if any deliveries failed due to encryption issues, if anyone attempted to interfere with connections, and whether your MTA-STS policy is working correctly. Without TLS-RPT, you are flying blind -- you would have no way to know if your MTA-STS policy is causing delivery problems or catching attacks.

Setting up TLS-RPT is straightforward. You add a TXT record at _smtp._tls.yourdomain.com that specifies where reports should be sent. It is strongly recommended to enable TLS-RPT alongside MTA-STS.

Each email security protocol protects a different part of the email process. Here is how they work together:

MTA-STS is more involved to set up than a simple DNS record. You need to configure a DNS TXT record, host a policy file on a specific subdomain over HTTPS with a valid certificate, and keep the policy updated when your mail servers change.

For businesses without a dedicated IT team, this can be a headache. The policy file must be served from a web server on a specific subdomain, the SSL certificate must be valid, and any misconfiguration can cause email delivery failures.

This is one reason MTA-STS adoption has been slower than SPF or DMARC, despite being an important layer of protection. Many businesses simply do not know it exists, and those that do often lack the technical resources to implement and maintain it correctly.