Email transmission between servers has historically been extremely insecure. A new draft internet standard called “SMTP Strict Transport Security” or “SMTP MTA STS” is aiming to help all email providers upgrade to a much more secure system for server-to-server mail transmission. This article lays out where we are currently in terms of email transmission security and how SMTP MTA STS will help.
Email servers (a.k.a. Mail Transmission Agents or “MTAs”) talk to each other using the Simple Mail Transmission Protocol (SMTP). This protocol, developed in 1982, originally lacked any hint of security. As a result, a lot of the email shooting around the internet is still transmitted in plain text. Its easily eavesdropped on, easily modified, untrusted and not private.
Back in 2002, an extension to SMTP called “STARTTLS” was standardized. This extension permitted servers to “upgrade” SMTP communications from plain text to an encrypted TLS-secured channel, when both servers supported compatible levels of TLS. This process is known as SMTP TLS. In principle, this security addition was really great. The “TLS” used is the same encryption method used by your web browsers to talk to secure web sites (e.g., banks, Amazon, your email provider, etc.). Your web browsers do relatively good job making sure that connections to these secure sites are safe. I.e., they seek to ensure that there is encryption, that the encryption is sufficiently strong, and that there is no one actively eavesdropping on your connections.
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