SMTP2S

SMTP-to-Storage

A simple component that mimics an SMTP server but stores e-mail data instead of actually submitting it.

This was built considering usage in tests or to relay messages from systems that only implement SMTP when you need the integration to be done via other means.

Features:

• Multiple message storage strategies:
  • S3
  • Local
• Basic ACL functionality,
• Structured logging formats.
• Metric exposure using OpenTelemetry.

Use Cases

What is stored

Every message is stored inside a folder with a dedicated execution ID (a simple ULID). Each execution folder will contain the following:

🔎 Message Metadata (metadata.json)

{
  "client": "[127.0.0.1]",
  "authenticated_user": "test@localhost.com",
  "from": "test@teste.com",
  "recipients": [
    "foo@bar.com"
  ],
  "to": [
    "foo@bar.com"
  ],
  "cc": [],
  "bcc": [],
  "subject": "teste",
  "date": "2025-09-11T22:43:34-03:00",
  "message_id": "6c2e0c6c-9535-4ae1-a920-3a6ffa036af5@teste.com"
}

✉️ An HTML message body (body.html)

📁 Attachments (in a dedicated attachment folder)

The file looks something like this:

storage-folder-or-s3-bucket/
├─ 01K4XSR779D9D9BTTVS7BBBRT3/
│  ├─ attachments/
│  |  ├─ file1.pdf
│  ├─ body.html
│  ├─ metadata.json
├─ 01K4XSRBY03MGFPQ3N6G0JW5ME/
│  ├─ attachments/
│  |  ├─ diagram-x.svg
│  |  ├─ flow-example.mp4
│  ├─ body.html
│  ├─ metadata.json
├─ 01K4XSRESDDW48DGZSN2KWB81D/
   ├─ attachments/
   ├─ body.html
   ├─ metadata.json

Get Started - With Docker

Docker Compose

version: "3.9"
services:
  smtp2s:
    image: incognitowski/smtp2s
    container_name: smtp2s
    network_mode: host
    volumes:
      - ./sample-configs:/configs
      - ./logs:/logs
    ports:
      - "8080:8080"
      - "9090:9090"
    command: ["--config-file=/configs/local-storage-config.json", "--log-level=INFO"]

Learn more about how to configure smtp2s in the guide below

Docker CLI

docker run -d \
  --name smtp2s \
  --network host \
  -v $(pwd)/sample-configs:/configs \
  -v $(pwd)/logs:/logs \
  incognitowski/smtp2s \
  --config-file=/configs/local-storage-config.json \
  --log-level=INFO

Get Started - Running & Configuring

Running with local storage

cargo run -- --config-file=sample-configs/local-storage-config.json

Running with S3 based storage

docker compose up -d
cargo run -- --config-file=sample-configs/s3-config.json

Controlling Log Level using --log-level

# Define log level based on https://docs.rs/tracing/latest/tracing/struct.Level.html
cargo run -- --config-file={config-file} --log-level=DEBUG

Default log level is INFO

Controlling logging format on an output-basis

cargo run -- --config-file={config-file} --stdout-log-kind=pretty --file-log-kind=json

Default for stdout is pretty, and for file-based is json

Controlling file based log directory

cargo run -- --config-file={config-file} --file-log-dir=/logs/smtp2s

Default is logs

config-file structure

{
    // The port smtp2s will be server on
    "port": 8080,
    // Port to expose metrics, may be null, in that case metrics won't be exposed
    "metrics_port": 9090,
    // --- Strategies ---
    // S3 - Requires a bucket name and an optional override_aws_endpoint
    "strategy": {
        "type": "S3",
        "bucket_name": "smtp2s-data-storage",
        "override_aws_endpoint": "http://localhost:4566"
    },
    // Local - Requires a base path to store files
    "strategy": {
        "type": "Local",
        "base_path": "./local-storage"
    },
    // List of addresses allowed to submit e-mails, or "*" for any.
    "allowed_addresses": [
        "*"
    ]
}