What Are SSL, TLS, & STARTTLS Email Encryption?. Understand how SSL, TLS, and STARTTLS protocols encrypt email communications, protecting data during transmission.​

• SSL, TLS, and STARTTLS are core protocols that secure email in transit by encrypting the communication channel.
• TLS is the modern, more secure successor to SSL, offering stronger protection and updated cipher suites.
• STARTTLS upgrades an existing insecure connection to a secure one and works with both SSL and TLS.
• Email encryption prevents attackers from reading intercepted messages, passwords, and sensitive data.
• The SSL/TLS handshake verifies the mail server’s identity before exchanging encryption keys.
• Opportunistic TLS encrypts messages whenever the receiving server supports it, improving security without breaking delivery.
• Using TLS alongside email authentication (SPF, DKIM, DMARC) strengthens overall email integrity.
• SparkPost uses HTTPS and TLS for inbound API calls, STARTTLS for SMTP, and opportunistic TLS for outbound mail.

• What is the difference between SSL and TLS?TLS is the newer, more secure version of SSL. Both encrypt data in transit, but TLS includes updated algorithms and improved protections.
• What does STARTTLS do?It tells the mail server to upgrade an existing insecure connection to a secure, encrypted one using SSL or TLS.
• Why is email encryption important?Because unencrypted email can be intercepted, exposing passwords, personal data, and sensitive information to attackers.
• How does the SSL/TLS handshake work?The client and server negotiate protocol versions, exchange certificates, verify identity, and agree on encryption keys before sending encrypted data.
• Is TLS required for all email delivery?Not always. Opportunistic TLS encrypts email when possible but still sends messages if the receiving server doesn’t support TLS.
• Does STARTTLS only work with TLS?No. Despite the name, STARTTLS works with both SSL and TLS.
• How does SparkPost support email encryption?SparkPost uses HTTPS/TLS for inbound API calls, supports STARTTLS for incoming SMTP, and uses opportunistic TLS for outbound email.

SSL, TLS, and STARTTLS refer to standard protocols used to secure email transmissions.

SSL (Secure Sockets Layer) and its successor, Transport Layer Security (TLS), provide a way to encrypt a communication channel between two computers over the Internet. In most cases, the terms SSL and TLS can be used interchangeably unless you’re referring to a specific version of the protocol.

Because TLS and SSL are application-layer protocols, senders and receivers need to know that they are being used to encrypt emails during transit. That’s where STARTTLS comes into play.

STARTTLS is an email protocol command that tells an email server that an email client, including an email client running in a web browser, wants to turn an existing insecure connection into a secure one. (By the way, the use of “TLS” in the STARTTLS command name does not mean that it only works with the TLS security protocol. It works with SSL too.)

Protocol

What it is

What it does

Why it matters

SSL

Older encryption protocol

Encrypts email in transit

Outdated but foundational; replaced by TLS

TLS

Modern version of SSL

Provides stronger, up-to-date encryption

Preferred standard for secure email delivery

STARTTLS

Command used in email protocols

Upgrades an insecure connection to a secure SSL/TLS one

Enables encryption even when the original connection is plain text

How does SSL work? When an email client sends and receives email, it uses TCP (Transmission Control Protocol) via the transport layer to initiate a “handshake” with the email server. During that basic setup process, the email client tells the email server which version of SSL or TLS it’s running and what cipher suites (a combination of processes used to negotiate security settings) and compression methods it wants to use.

After the setup is finished, the email server verifies its identity to the email client by sending a certificate that is trusted by the user’s software, or by a third party trusted by it. Doing so ensures that the email client isn’t sending messages to an imposter. Once the client knows it can trust the server, a key is exchanged between the two, which allows all messages sent and received to be encrypted.

Why should you care about SSL or TLS? It’s important to use SSL or TLS with your email setup because unsecure email is a common attack vector for the bad guys. Anyone who intercepts encrypted emails is left with garbage text that they can’t do anything with, because only the email server and client have the keys to decode the messages.

This is key for ensuring the protection of user names, passwords, personal details, and other sensitive information that’s often found in emails. If an attacker discovers a weakness, they will exploit it for as long as they can and mine data that will be sold on the black market.

TLS is the preferred encryption method because it’s newer and offers more robust security features than SSL does.

It’s also a good idea to combine TLS-based email encryption with email authentication to ensure the integrity of email messages.

Read additional SSL, TLS, and STARTTLS resources Here are some resources that will help you dig deeper into SSL, TLS, and STARTTLS:

• Wikipedia’s entry on SSL and TLS: This is a good overview of the history of the encryption protocols and their technical details.
• Wikipedia’s entry on Opportunistic TLS, or STARTTLS