Bridging the Gap: Enforcing TLS 1.3 for Legacy On-Premises Devices in Email Delivery

In the world of email delivery, security is paramount. As protocols and standards evolve, staying up-to-date with the latest encryption methods is essential to protect sensitive information. TLS 1.3, the latest version of the Transport Layer Security protocol, provides enhanced security and performance compared to its predecessors. However, not all systems are ready to make the leap, especially legacy on-premises devices that only support plaintext or outdated protocols like TLS 1.1.

This creates a dilemma for businesses: How do you ensure secure email delivery while maintaining compatibility with older devices? In this article, we’ll explore the problem, propose a solution, and provide a configuration guide for stunnel, a tool that can bridge the gap between legacy devices and modern, secure communication.

The Problem: Legacy Devices and Modern Security Standards

Many organizations rely on older devices that lack support for modern encryption standards. These devices may only support:

• Plaintext communication, which is highly insecure and vulnerable to interception.
• Outdated encryption protocols like TLS 1.1, which are no longer considered safe.

At the same time, email relay services like Mimecast require secure communication, with a preference for TLS 1.2 or TLS 1.3. This creates a compatibility issue: legacy devices are unable to connect securely to email smarthosts without additional intervention.

The Solution: Stunnel as a Secure Proxy

To solve this problem, we can use stunnel, a lightweight tool designed to act as a secure proxy. Stunnel allows us to:

1. Accept plaintext or older TLS connections locally from legacy devices.
2. Securely encrypt and enforce TLS 1.3 for outgoing connections to Mimecast.

This approach ensures that even older devices can continue to function while protecting your data during email delivery over the internet. Below, we’ll walk you through a step-by-step guide to configure stunnel for this purpose.

Step-by-Step Configuration

Here’s how to configure stunnel to accept plaintext connections locally and enforce TLS 1.3 when sending emails to Mimecast’s outbound SMTP servers (xx-smtp-outbound-1.mimecast.com and xx-smtp-outbound-2.mimecast.com).

1. Install Stunnel

First, install stunnel on your server. On a Linux-based system, you can install it using the following commands:

sudo apt update
sudo apt install stunnel4

2. Generate a Stunnel Certificate

Stunnel requires a certificate for secure communication. You can generate a self-signed certificate by running:

openssl req -new -x509 -days 365 -nodes -out stunnel.pem -keyout stunnel.pem
chmod 600 stunnel.pem

Move the stunnel.pem file to a secure location, such as /etc/stunnel/.

3. Configure Stunnel

Create a configuration file for stunnel. Name it stunnel.conf and place it in /etc/stunnel/. Here’s an example configuration tailored for Mimecast:

# Stunnel Configuration

# Use the generated certificate
cert = /etc/stunnel/stunnel.pem

# Enforce TLS 1.3 for outgoing connections
sslVersion = TLSv1.3

# Debugging (set to 5 for detailed logs during troubleshooting)
debug = 3
output = /var/log/stunnel.log

# Service 1: Connect to Mimecast's primary SMTP server
[xx-smtp-outbound-1]
client = yes
accept = 127.0.0.1:2525  # Local port for plaintext connections
connect = xx-smtp-outbound-1.mimecast.com:587

# Service 2: Connect to Mimecast's secondary SMTP server
[xx-smtp-outbound-2]
client = yes
accept = 127.0.0.1:2526  # Local port for plaintext connections
connect = xx-smtp-outbound-2.mimecast.com:587

4. Start Stunnel

Enable and start the stunnel service:

sudo systemctl enable stunnel4
sudo systemctl start stunnel4

To verify that stunnel is running and listening on the specified ports (2525 and 2526), use the following command:

sudo netstat -tuln | grep stunnel

5. Update Your Devices

Configure your legacy on-premises devices to send emails to 127.0.0.1 on ports 2525 (for the primary server) or 2526 (for the secondary server). These local connections will be forwarded by stunnel to Mimecast with TLS 1.3 encryption.

6. Test the Configuration

To ensure everything is working as expected, you can test the local connection using telnet:

telnet 127.0.0.1 2525

Once connected, send a test email through your legacy device or email client configured to use the stunnel proxy. Verify that the email is successfully delivered.

Why Choose Stunnel?

Using stunnel provides several key benefits:

• Compatibility: Legacy devices can continue to use plaintext or older TLS protocols without modification.
• Security: Outgoing connections to Mimecast are encrypted with TLS 1.3, ensuring compliance with modern security standards.
• Simplicity: Stunnel is lightweight, easy to configure, and doesn’t require significant changes to your infrastructure.

Final Thoughts

In today’s cybersecurity landscape, it’s critical to balance compatibility with legacy systems and modern security requirements. Tools like stunnel provide an elegant solution, enabling you to secure your email delivery while accommodating older devices.

By following the steps outlined in this article, you can enforce TLS 1.3 for your email delivery via Mimecast, ensuring both security and compliance. If you have any questions or run into challenges with this setup, feel free to leave a comment below or reach out—we’re here to help!

Let’s keep email delivery secure, reliable, and future-proof.