E-MailRelay explained: the free, open-source C++ SMTP proxy and store-and-forward mail server for Linux and Windows, with features and setup guidance.
Category
Communication
Pricing
Open Source, from Free
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Last updated
July 18, 2026
WindowsOpen SourceSelf-Hosted
Overview
E-MailRelay (EmailRelay) is a free, open-source SMTP proxy and store-and-forward mail transfer agent written in C++, distributed through SourceForge for Linux and Windows.
It is designed as a narrow, single-purpose relay rather than a full mail server platform, focusing on receiving, storing, filtering, and forwarding SMTP mail.
Key Features
E-MailRelay runs as a single non-blocking process, acting simultaneously as an SMTP storage daemon, an SMTP forwarding agent, and a POP3 server for local mail clients.
It supports piping messages through external filtering programs such as SpamAssassin, DNSBL-based connection blocking, and configurable routing to a fixed smarthost or custom destinations.
Pricing
E-MailRelay is completely free and open source, with no paid tier, licensing fee, or commercial edition.
It is distributed as source code and pre-built binaries through its SourceForge project page at no cost.
Key Features
SMTP storage daemon — Accepts and stores incoming email messages for later forwarding or retrieval.
SMTP forwarding agent — Relays stored messages onward to a designated smarthost or performs custom routing to other destinations.
POP3 server — Serves stored messages to local email client software using the POP3 protocol.
External filter hooks — Pipes messages through a user-defined program, allowing integration with spam filters like SpamAssassin or custom editing scripts.
DNSBL connection blocking — Rejects connections from known spam sources using DNS-based blocklists.
Non-blocking I/O architecture — Runs as a single lightweight process using the same event-driven model as Squid and nginx for efficient resource usage.
Cross-platform support — Runs on both Linux and Windows, with a graphical installation wizard available for Windows deployments.
Lightweight, dependency-light codebase — Written in C++ with minimal dependencies, making it easy to compile and run on constrained or older hardware.
Pros & Cons
Pros
Completely free and open source with no licensing costs
Very lightweight and resource-efficient compared to full mail server suites
Flexible filtering hook lets administrators plug in SpamAssassin or custom scripts
Runs on both Linux and Windows with a graphical installer for Windows users
Cons
Narrower feature set than full mail server platforms like Postfix or Exim
No official commercial support, so troubleshooting relies on documentation and community resources
Configuration is command-line and config-file driven, which requires technical comfort
Smaller, less active community compared to mainstream MTA projects
Pricing
Open Source Free N/A
Frequently Asked Questions
Is EmailRelay (E-MailRelay) free?
Yes, E-MailRelay is completely free and open source, distributed through SourceForge with no licensing fee or paid tier.
What platforms does E-MailRelay support?
E-MailRelay runs on both Linux and Windows, and includes a graphical installation wizard for Windows deployments.
What does E-MailRelay actually do?
It acts as an SMTP proxy and store-and-forward mail transfer agent, receiving, storing, optionally filtering, and forwarding email to another mail server, while also serving stored mail to local clients over POP3.
Can E-MailRelay filter spam?
Yes, it can pipe incoming messages through an external program such as SpamAssassin, and supports DNSBL-based blocking to reject connections from known spam sources.
How is E-MailRelay different from Postfix?
E-MailRelay is a narrower, lightweight SMTP proxy and relay rather than a full-featured production mail server like Postfix; it focuses specifically on storing, filtering, and forwarding mail rather than complete mailbox and delivery infrastructure.
Who maintains E-MailRelay?
E-MailRelay is maintained as an open-source project hosted on SourceForge, without a dedicated commercial company behind it.