SMTP Relay

This Docker image provides a lightweight SMTP relay service using msmtp for sending emails via SMTP server that supports STARTTLS and authentication.

Features

• Lightweight Alpine-based image
• STARTTLS and TLS encryption enabled by default
• Configurable via environment variables
• Works with Office 365 and other SMTP providers
• Restricts allowed networks for relay to improve security

Environment Variables

Variable	Description	Example
SMTP_MESSAGE_SIZE_LIMIT	Maximum email size (in bytes) allowed by the relay	52428800
SMTP_RELAY_HOST	Upstream SMTP relay host and port	smtp.office365.com:587
SMTP_RELAY_MYHOSTNAME	Hostname to be used in the relay's HELO/EHLO	relay.example.com
SMTP_RELAY_MYNETWORKS	Comma-separated list of networks allowed to relay emails	10.0.0.0/8, 192.168.0.0/16
SMTP_RELAY_USERNAME	SMTP relay authentication username (usually your email address for Office 365)	[email protected]
SMTP_RELAY_PASSWORD	SMTP relay authentication password or application-specific password	yourpassword

---

Example docker run

docker run -d --name smtp-relay \
  -e SMTP_MESSAGE_SIZE_LIMIT=52428800 \
  -e SMTP_RELAY_HOST="smtp.office365.com:587" \
  -e SMTP_RELAY_MYHOSTNAME="relay.example.com" \
  -e SMTP_RELAY_MYNETWORKS="10.0.0.0/8, 192.168.0.0/16" \
  -e SMTP_RELAY_USERNAME="[email protected]" \
  -e SMTP_RELAY_PASSWORD="yourpassword" \
  -p 2525:2525 \
  ghcr.io/cossth/smtp-proxy:main

---

Example docker-compose.yml

Please see docker-compose.yaml

version: '3.8'
services:
  smtp-relay:
    image: ghcr.io/cossth/smtp-proxy:main
    container_name: smtp-relay
    ports:
      - "2525:2525"
    environment:
      SMTP_MESSAGE_SIZE_LIMIT: 52428800
      SMTP_RELAY_HOST: "smtp.office365.com:587"
      SMTP_RELAY_MYHOSTNAME: "relay.example.com"
      SMTP_RELAY_MYNETWORKS: "10.0.0.0/8, 192.168.0.0/16"
      SMTP_RELAY_USERNAME: "[email protected]"
      SMTP_RELAY_PASSWORD: "yourpassword"
    restart: unless-stopped

Example Flux/Kustomization

You can add below configs in your cluster to deploy the smtp-relay.

Kustomization Files

apiVersion: source.toolkit.fluxcd.io/v1
kind: GitRepository
metadata:
  name: cossth
spec:
  interval: 24h
  url: https://github.com/cossth/smtp-proxy
  ref:
    branch: main
---
apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
  name: smtp-proxy
  namespace: flux-system
spec:
  interval: 10m
  path: ./example/kubernetes
  prune: true
  sourceRef:
    kind: GitRepository
    name: cossth

Security Notes

• Restrict SMTP_RELAY_MYNETWORKS to trusted IP ranges (e.g., your Kubernetes cluster network or your Docker network).
• Use application-specific passwords where possible.

Testing the Relay

You can test by connecting via telnet or using a mail client:

Please note your ip should be allowed in SMTP_RELAY_MYNETWORKS.

telnet localhost 25
EHLO test
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
Subject: Test Email
This is a test.
.
QUIT