What is MTA-STS? Setting the right MTA STS policy
Posted: Tue Apr 22, 2025 10:51 am
SMTP Mail Transfer Agent - Strict Transport Security (MTA-STS) is a widely known Internet standard that helps improve the security of connections between SMTP (Simple Mail Transfer Protocol) servers. MTA-STS addresses existing SMTP email security by performing TLS encryption during transmission.
History and Origins of MTA-STS
When SMTP was first specified in 1982, it did not contain any buy bulk sms service mechanisms to provide security at the transport level to secure communications between mail transfer agents. However, in 1999, the STARTTLS command was added to SMTP, which in turn supported email encryption between servers, providing the ability to convert a non-secure connection to a secure connection encrypted using the TLS protocol.
In this case, you must be wondering whether SMTP uses STARTTLS to secure the connection between servers, why you need to turn to MTA-STS, and what its role is. Let’s find out in the following sections of this blog!
What is MTA-STS? (Mail Transfer Agent Strict Transport Security - Explained)
MTA-STS is a security standard that ensures secure transmission of email over encrypted SMTP connections. The acronym MTA stands for Message Transfer Agent, which is a program that transfers email messages between computers. The acronym STS stands for Strict Transport Security, which is the protocol used to implement the standard. An MTA-STS-aware mail transfer agent (MTA) or secure message transfer agent (SMTA) operates in accordance with this specification to provide a secure end-to-end channel for sending email over an insecure network.
The MTA-STS protocol allows an SMTP client to verify the identity of a server and ensure that it is not connecting to an imposter by requiring the server to provide its certificate fingerprint in the TLS handshake. The client then verifies that certificate against a trust store containing known server certificates.
Introduction to MTA-STS Email Security
MTA-STS was introduced to fill the security gap in SMTP communications. As a security standard, MTA-STS ensures the secure transmission of email messages over encrypted SMTP connections.
The abbreviation MTA stands for Message Transfer Agent, which is a program that transfers email messages between computers. The abbreviation STS stands for Strict Transport Security, which is the protocol used to implement this standard. An MTA-STS-aware message transfer agent (MTA) or secure message transfer agent (SMTA) operating in accordance with this specification provides a secure end-to-end channel for sending email over an insecure network.
The MTA-STS protocol allows an SMTP client to verify the identity of a server and ensure that it is not connecting to an imposter by requiring the server to provide its certificate fingerprint in the TLS handshake. The client then verifies that certificate against a trust store containing known server certificates.
The need to move to mandatory TLS encryption
STARTTLS is not perfect and it fails to address two major issues: First, it is an optional measure, so STARTTLS cannot prevent man-in-the-middle (MITM) attacks. This is because a MITM attacker can easily modify the connection and prevent encryption updates. Its second problem is that even if STARTTLS is implemented, it cannot verify the identity of the sending server like SMTP does. SMTP mail servers do not verify certificates.
Although most outgoing emails are now encrypted using Transport Layer Security (TLS), attackers can still intercept and tamper with emails before they are encrypted. If you send emails over a secure connection, your data could be compromised, modified, or tampered with by cyber attackers.
History and Origins of MTA-STS
When SMTP was first specified in 1982, it did not contain any buy bulk sms service mechanisms to provide security at the transport level to secure communications between mail transfer agents. However, in 1999, the STARTTLS command was added to SMTP, which in turn supported email encryption between servers, providing the ability to convert a non-secure connection to a secure connection encrypted using the TLS protocol.
In this case, you must be wondering whether SMTP uses STARTTLS to secure the connection between servers, why you need to turn to MTA-STS, and what its role is. Let’s find out in the following sections of this blog!
What is MTA-STS? (Mail Transfer Agent Strict Transport Security - Explained)
MTA-STS is a security standard that ensures secure transmission of email over encrypted SMTP connections. The acronym MTA stands for Message Transfer Agent, which is a program that transfers email messages between computers. The acronym STS stands for Strict Transport Security, which is the protocol used to implement the standard. An MTA-STS-aware mail transfer agent (MTA) or secure message transfer agent (SMTA) operates in accordance with this specification to provide a secure end-to-end channel for sending email over an insecure network.
The MTA-STS protocol allows an SMTP client to verify the identity of a server and ensure that it is not connecting to an imposter by requiring the server to provide its certificate fingerprint in the TLS handshake. The client then verifies that certificate against a trust store containing known server certificates.
Introduction to MTA-STS Email Security
MTA-STS was introduced to fill the security gap in SMTP communications. As a security standard, MTA-STS ensures the secure transmission of email messages over encrypted SMTP connections.
The abbreviation MTA stands for Message Transfer Agent, which is a program that transfers email messages between computers. The abbreviation STS stands for Strict Transport Security, which is the protocol used to implement this standard. An MTA-STS-aware message transfer agent (MTA) or secure message transfer agent (SMTA) operating in accordance with this specification provides a secure end-to-end channel for sending email over an insecure network.
The MTA-STS protocol allows an SMTP client to verify the identity of a server and ensure that it is not connecting to an imposter by requiring the server to provide its certificate fingerprint in the TLS handshake. The client then verifies that certificate against a trust store containing known server certificates.
The need to move to mandatory TLS encryption
STARTTLS is not perfect and it fails to address two major issues: First, it is an optional measure, so STARTTLS cannot prevent man-in-the-middle (MITM) attacks. This is because a MITM attacker can easily modify the connection and prevent encryption updates. Its second problem is that even if STARTTLS is implemented, it cannot verify the identity of the sending server like SMTP does. SMTP mail servers do not verify certificates.
Although most outgoing emails are now encrypted using Transport Layer Security (TLS), attackers can still intercept and tamper with emails before they are encrypted. If you send emails over a secure connection, your data could be compromised, modified, or tampered with by cyber attackers.