Glossary

Transport Layer Security (TLS)

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Transport Layer Security (TLS)

Transport Layer Security (TLS) is a cryptographic protocol that encrypts data in transit between email servers, protecting emails from eavesdropping and tampering while they’re on the move. It also provides server authentication, helping verify that messages are exchanged between legitimate parties.  

TLS encrypts email content during transmission (not end-to-end, but server-to-server), prevents man-in-the-middle attacks during SMTP relay, and authenticates servers to establish a secure connection. 

It does not encrypt the content on the sender’s or recipient’s device. 

TLS is not the same as end-to-end encryption, which keeps the email content private from sender to recipient, and even from the email provider itself. 

Related protocols include: 

  • SSL (Secure Sockets Layer): Outdated and deprecated predecessor to TLS, and still mentioned occasionally, but not recommended. 
  • End-to-end encrpytion (Like PGP or S/MIME): Encrypts the entire message from sender to recipient so that only the intended recipient can read it, even if the email is intercepted or the provider is compromised. 
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