A message is not 'transmitted via MTA-STS', it is transmitted via SMTP that uses TLS (SSL) to secure the connection between mailservers. MTA-STS [0] is a metho

A message is not 'transmitted via MTA-STS', it is transmitted via SMTP that uses TLS (SSL) to secure the connection between mailservers. MTA-STS [0] is a method to publish a policy on whether email servers (known as Mail Transfer Agents, or MTA's) are allowed to use insecure (non-TLS) connections for email from your domain. MTA-STS is needed because the system to deliver email over the internet (SMTP) has a fallback method where it will switch to an unencrypted connection if the encrypted connection fails for some reason. By blocking a encrypted connection in the middle, an attacker can force a mailserver to fall-back to a non-encrypted connection. This way the attacker can intercept the message in transit. This is known as a downgrade attack. Some governments are known for doing this. So, MTA-STS prevents downgrade attacks (to a certain extend, because the MTA's that your email passes through must support it). There is also a new proposed standard called TLS-RPT [1] that allows for reporting the usage of TLS, so you know if your MTA-STS policy caused an email delivery to fail. At Mailhardener [2] we are working on hosted MTA-STS and TLS-RPT integration (both features are still in closed beta for now). It's great to see Gmail adopting MTA-STS, as this will accelerate adoption globally. [0] https://tools.ietf.org/html/rfc8461 https://tools.ietf.org/html/rfc8461 [1] https://tools.ietf.org/html/rfc8460 https://tools.ietf.org/html/rfc8460 [2] https://www.mailhardener.com https://www.mailhardener.com