What Is STARTTLS in SMTP?

What Is STARTTLS in SMTP?

STARTTLS is an SMTP command that upgrades a plain text connection to an encrypted TLS (Transport Layer Security) connection. It allows SMTP servers and clients to start with an unencrypted connection and then negotiate encryption, providing security while maintaining backward compatibility.

STARTTLS is used on port 587 (the submission port) and is the recommended method for securing SMTP connections. It encrypts email transmission, protects authentication credentials, and prevents eavesdropping on email communications.

Test STARTTLS support on your SMTP server using our SMTP server test tool to verify encryption is properly configured.

How STARTTLS Works

1. Initial Connection

Client connects to SMTP server on port 587 using a plain text connection. The server responds with its capabilities.

2. STARTTLS Command

If the server supports STARTTLS, the client sends the STARTTLS command to request encryption upgrade.

3. Encryption Negotiation

Server and client negotiate TLS encryption parameters, including TLS version and cipher suites.

4. TLS Handshake

TLS handshake occurs, establishing encrypted connection. Server presents SSL/TLS certificate for verification.

5. Encrypted Communication

All subsequent communication (including authentication and email transmission) occurs over the encrypted TLS connection.

6. Backward Compatibility

If STARTTLS is not supported, the connection can continue unencrypted (though this is not recommended for security).

Security Benefits

1. Encryption

STARTTLS encrypts email content during transmission, preventing interception and eavesdropping.

2. Authentication Protection

Encrypts authentication credentials (username and password), preventing credential theft.

3. Data Integrity

TLS ensures data integrity, detecting tampering or modification during transmission.

4. Man-in-the-Middle Prevention

Certificate validation helps prevent man-in-the-middle attacks by verifying server identity.

5. Compliance

STARTTLS helps meet security and compliance requirements for email transmission.

STARTTLS vs SSL/TLS

STARTTLS (Port 587)

• Starts with plain text connection
• Upgrades to encrypted TLS connection
• Backward compatible
• Standard for email submission
• Recommended for most use cases

Implicit SSL/TLS (Port 465)

• Encryption from connection start
• No plain text phase
• Requires SSL/TLS from beginning
• Alternative to STARTTLS
• Still widely supported

Which to Use

STARTTLS on port 587 is generally recommended as the standard for email submission. Port 465 with implicit SSL/TLS is an alternative that's still widely supported.

Testing STARTTLS

Testing STARTTLS support ensures your SMTP server properly supports encrypted connections:

1. Server Capability Check

Verify the server advertises STARTTLS support in its capability response (EHLO command).

2. STARTTLS Command Test

Test that the STARTTLS command is accepted and successfully upgrades the connection.

3. TLS Handshake Verification

Verify TLS handshake completes successfully and encryption is established.

4. Certificate Validation

Check SSL/TLS certificate validity, hostname matching, and certificate chain.

5. Encryption Testing

Verify that communication after STARTTLS is actually encrypted and secure.

Use our SMTP server test tool to verify STARTTLS support, test encryption, and validate certificate configuration.