What is the difference between ESMTPS and ESMTPSA in email headers?

ESMTPS and ESMTPSA are extensions to the SMTP protocol related to secure email transmission. ESMTPS indicates that Transport Layer Security (TLS) was used to encrypt the email. ESMTPSA signifies that both TLS encryption and SMTP Authentication were used. Authentication generally implies a higher level of trust, ensuring that the sender has authenticated with the mail server. STARTTLS is now the preferred method for securing SMTP connections, where a plain text connection is upgraded to a secure TLS connection. Older methods such as SMTPS are deprecated. RFCs and other documentation outline the implementation of these security measures and the role of authentication in verifying users. In practice, internal handoffs often use ESMTPSA, while MTA to MX communication relies on ESMTPS.

ESMTPS and ESMTPSA are extensions to the SMTP protocol that enhance email security. ESMTPS signifies that Transport Layer Security (TLS) was used to encrypt the email transmission. ESMTPSA, in addition to TLS encryption, indicates that authentication was also used, implying a more secure connection as the sender authenticated with the mail server. While SMTPS (SMTP Secure) was an older, now deprecated method, STARTTLS is currently preferred, allowing for a plain text connection to be upgraded to a secure one using TLS. Implementing TLS/SSL encryption is crucial for securing SMTP to protect email content during transmission. The use of authentication ensures that only authorized users can send emails through the server.

In email communication, internal handoffs within a mail system often use ESMTPSA, indicating both TLS encryption and authentication. The transfer from the Mail Transfer Agent (MTA) to the receiving Mail Exchanger (MX) typically uses ESMTPS, employing TLS encryption alone. Modern email security relies on opportunistic TLS via STARTTLS, where the mail server announces TLS support for secure communication.

The RFC documentation outlines that Extended SMTP (ESMTP) provides the framework for extensions like TLS and authentication. RFC documents explain the implementation of TLS to secure SMTP connections and the addition of authentication mechanisms where clients log in to mail servers. STARTTLS, described in Microsoft documentation, allows upgrading plain text SMTP connections to encrypted ones. These RFC documents collectively provide the underlying specifications for achieving secure email transport and authentication relevant to understanding ESMTPS and ESMTPSA.