Mail Transfer Agent Strict Transport Security (MTA-STS) is a standard that enables the encryption of messages being sent between two mail servers. It specifies to sending servers that emails can only be sent over a Transport Layer Security (TLS) encrypted connection which prevents emails from being intercepted by cybercriminals.

The Simple Mail Transfer Protocol (SMTP) alone does not provide security, making it vulnerable to malicious attacks such as man-in-the-middle attacks. A man-in-the-middle attack is where communication between two servers is intercepted and possibly changed without detection by the recipient.

In addition, encryption is optional in SMTP, which means that emails can be sent in plaintext. If a plaintext email was intercepted in transit, it could easily be read and manipulated. Without MTA-STS, an attacker can intercept the communication and force the sending service to send the message in plain text. 

By enabling MTA-STS, a TLS connection is required which ensures encryption and keeps your emails private. The MTA-STS standard is so critical to improving the security of SMTP that it has widespread support among major mail service providers such as Google and Microsoft.

For more technical details, visit the MTA-STS and TLS chapter in our Technical Configuration Guide.