EmailRelay

E‑MailRelay is a lightweight mail server that implements SMTP store‑and‑forward and provides POP access to the spooled messages. It can forward mail to a fixed smarthost or perform DNS MX lookups for routing, and it supports delivery to maildir mailboxes with optional SpamAssassin filtering and DNSBL blocking. The software runs as a single non‑blocking I/O process, similar to Squid or nginx, which gives it good scalability while keeping dependencies minimal.

The server is designed to be policy‑free, allowing users to add their own scripts for address validation, retry handling, bounce processing, and custom spam filters. Configuration is performed via command‑line options; it can be started directly from a zip archive on Windows or as a background daemon on Linux, with separate modes for storage (--as‑server) and client forwarding (--as‑client). Features such as periodic polling, immediate forwarding on disconnect, and custom filter programs are available through simple flags.

Because it is self‑hostable, open‑source under GPL‑3.0, and requires no subscription, it is suitable for individuals or small organizations that need a simple, configurable SMTP/POP3 solution on either Windows or Linux platforms.