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Email Protocols

TLS(Transport Layer Security)

Encryption protocol that secures email transmission between servers.

Definition

TLS (Transport Layer Security) is a cryptographic protocol that encrypts data transmitted between email servers and clients. When emails are sent with TLS, the content is protected from eavesdropping and tampering during transit. TLS is the successor to SSL and is the current standard for secure email transmission. Major email providers like Gmail and Microsoft 365 require TLS for incoming connections.

Why It Matters

TLS protects your email content and subscriber data during transmission. Many email providers now require TLS for delivery, and some recipients' servers will reject emails sent without encryption. Using TLS also builds trust with privacy-conscious subscribers. Google even shows a warning icon in Gmail for emails received without TLS encryption.

How It Works

TLS works through a handshake process where servers exchange certificates and negotiate encryption parameters. STARTTLS upgrades an existing connection to encrypted, while implicit TLS starts encrypted from the beginning. Once established, all data is encrypted before transmission and decrypted upon receipt. The encryption strength depends on the cipher suites both servers support.

Example

When you send a newsletter through Sequenzy to a Gmail recipient, here is what happens:

  1. 1Sequenzy's server connects to Gmail's server on port 25
  2. 2Gmail responds: "I support STARTTLS"
  3. 3Sequenzy initiates TLS and they exchange certificates
  4. 4They agree on TLS 1.3 with a strong cipher
  5. 5Your email is encrypted and sent through the encrypted tunnel
  6. 6Gmail decrypts the email and delivers it

Anyone intercepting the traffic only sees scrambled data. In Gmail, your recipient sees a small lock icon indicating the email was delivered securely.

Best Practices

  • 1Always use TLS when sending emails through your ESP
  • 2Configure your sending domain with TLS 1.2 or higher (TLS 1.3 preferred)
  • 3Monitor for TLS failures in delivery logs as they may indicate configuration issues
  • 4Use opportunistic TLS for maximum deliverability while maintaining security
  • 5Verify your SSL/TLS certificates are valid and not expired

Frequently Asked Questions

TLS is the modern successor to SSL. While people often use the terms interchangeably, SSL is outdated and has known vulnerabilities. TLS 1.0 and 1.1 are also deprecated. Always use TLS 1.2 or TLS 1.3 for secure email transmission.

TLS encrypts email during transmission between servers, protecting it from interception in transit. However, once delivered, the email is decrypted and may be stored unencrypted on the recipient's server. For true end-to-end encryption, additional technologies like S/MIME or PGP are needed.

STARTTLS is a command that upgrades an existing unencrypted connection to an encrypted TLS connection. It allows servers to use the same port for both encrypted and unencrypted connections. Most SMTP connections use STARTTLS on port 587 or 25.