What happens when legacy hardware can no longer send email because modern cloud providers have tightened security? In this episode, we dive into Email Relay, a lightweight open-source mail proxy that helps old devices like scanners, switches, and monitoring systems continue to send alerts securely in a modern infrastructure.
Email Relay acts as a local middleman between legacy equipment and strict upstream providers such as Microsoft 365 or Google Workspace. Older devices can send simple, unauthenticated messages to Email Relay on the local network, and it then upgrades those messages with modern authentication, encryption, and routing before forwarding them onward. This makes it possible to preserve expensive hardware investments without replacing perfectly functional equipment just because email standards have changed.
We explore how Email Relay achieves this with a remarkably efficient architecture. Built in C++ and using a non-blocking I/O model, it can handle large volumes of mail traffic with minimal system resources - making it practical even on modest hardware. Its policy-free design also gives administrators full control over routing and filtering logic, rather than forcing them to work around hard-coded assumptions.
The episode also examines the platform’s flexible filter system, built-in tools for routing and splitting traffic, and integrations with SpamAssassin and address verifiers to protect against spam and abuse. Security features like TLS encryption, PAM authentication, and optional proxy or Tor routing make it suitable even for sensitive environments.
Finally, we look at real-world user reports that validate its stability and scale - including deployments that have run for years without crashing and others that process hundreds of thousands of emails per day.
If you’ve ever wondered whether a small, open-source tool can replace bloated proprietary mail infrastructure, this deep dive into Email Relay shows just how powerful a focused, efficient design can be.
