I Built a Lightweight SMTP-to-Mailgun Bridge in Go (In Just Two Days)

Описание и применение

Mailgun Passport — легковесный мост SMTP‑в‑Mailgun, написанный на Go, который позволяет старым приложениям, говорящим только по SMTP, отправлять письма через Mailgun без установки тяжёлого MTA.

Задача

Legacy applications that only speak SMTP cannot send mail through modern providers that block outbound port 25.

Результат

Mailgun Passport enables legacy SMTP clients to send mail through Mailgun in a lightweight, secure, and production‑ready way, eliminating the need for a full MTA.

Рабочий процесс

• 1. Run Mailgun Passport locally on 127.0.0.1:587.
• 2. Send SMTP messages to the local server (e.g., via swaks).
• 3. Passport parses the raw SMTP envelope and MIME body.
• 4. It reconciles envelope recipients with MIME headers, treating missing recipients as Bcc.
• 5. Passport forwards the message to Mailgun’s /v3/{domain}/messages endpoint via HTTP.
• 6. Systemd runs the service as a dedicated low‑privilege user with sandboxed capabilities.

Доказательства

If you've ever tried to plug a legacy application, an old monitoring script, or an enterprise tool into a modern email delivery service like Mailgun, you've probably hit a brick wall: your app only speaks SMTP, but many cloud providers block outbound port 25 entirely.

Источник

Mailgun Passport, a minimal SMTP-to-Mailgun bridge written in Go.

Источник

It already supports native RPM/DEB packaging, structured logging, flexible configuration via .env or systemd, and dual parsing/MIME modes.

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