What Is TLS (Transport Layer Security)?

TLS (Transport Layer Security) is a cryptographic protocol that secures communication between mail servers during email transmission. It ensures that messages sent over the internet cannot be easily intercepted, read, or modified by unauthorized parties. TLS encrypts the connection between sending and receiving servers, protecting the data in transit, including message headers, content, and attachments, from eavesdropping and tampering.

TLS is the successor to SSL (Secure Sockets Layer) and has become the standard for secure email transport. Most modern mail servers use TLS automatically when both sides support it, enabling secure transmission without requiring manual user intervention. While TLS does not authenticate the sender or guarantee message integrity like DKIM or DMARC, it plays a critical role in preventing unauthorized access during delivery.

How TLS Works in Email Transmission

TLS operates at the transport layer of the email delivery process, encrypting the session established between two mail transfer agents (MTAs). When one mail server connects to another via SMTP, it can upgrade the connection to a secure channel using the STARTTLS command.

Example of a secure SMTP handshake:

After this negotiation, both servers exchange digital certificates to verify each other’s identities and establish an encrypted session. All subsequent data, including message contents, is transmitted securely through that connection.

TLS uses asymmetric cryptography for key exchange (public and private keys) and symmetric encryption for message transmission. This hybrid approach provides strong security without significantly slowing down message delivery.

TLS Versions and Cipher Strength

Modern email systems support several versions of TLS, with each new iteration addressing vulnerabilities found in earlier versions. The most current and recommended version is TLS 1.3, offering improved encryption and faster handshakes. Older versions such as TLS 1.0 and 1.1 are deprecated due to security flaws and should not be used.

Best practices for secure TLS configuration include:

• Enforcing TLS 1.2 or higher for SMTP communication
• Disabling insecure ciphers and legacy SSL protocols
• Using certificates from trusted Certificate Authorities (CAs)
• Implementing MTA-STS to enforce secure mail transport
• Regularly renewing and monitoring TLS certificates

Without proper configuration, an attacker could intercept unencrypted connections through a downgrade attack, forcing communication to occur without encryption - a risk mitigated by MTA-STS and DANE (DNS-based Authentication of Named Entities).

Why TLS Is important for Email Security

TLS ensures the privacy and confidentiality of email messages while in transit between mail servers. Although it doesn’t authenticate senders or validate domains (tasks handled by SPF, DKIM, and DMARC), it prevents message exposure and manipulation during transfer.

Key benefits of TLS in email include:

• Prevents data interception and eavesdropping
• Protects credentials, attachments, and message content
• Reduces the risk of man-in-the-middle attacks
• Supports compliance with data protection regulations (e.g., GDPR, HIPAA)
• Builds user and recipient trust through secure transport

Without TLS, emails are transmitted in plaintext, allowing anyone with network access to read or alter their contents. For organizations handling sensitive or confidential data, TLS is essential to maintaining secure communications and protecting brand reputation.

TLS and DMARCeye

DMARCeye provides full visibility into your domain’s authentication and transport security posture, including TLS usage across all sending sources. The platform detects mail streams that fail to use encryption, identifies weak or expired certificates, and correlates TLS status with authentication outcomes for a complete view of message security.

By analyzing both DMARC reports and TLS connection data, DMARCeye helps organizations ensure that email delivery remains both authenticated and encrypted. This dual-layer insight strengthens domain protection, compliance, and trust across all communication channels.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.