genro-mail-proxy 0.5.0

Asynchronous email dispatcher microservice with scheduling, rate limiting, attachments, and a FastAPI REST API.

genro-mail-proxy

A microservice that decouples email delivery from your application.

What it does

genro-mail-proxy sits between your application and SMTP servers. Your application sends messages to the proxy via REST API; the proxy handles delivery with:

• Persistent queue: Messages are stored in SQLite or PostgreSQL and survive restarts
• Automatic retry: Failed deliveries are retried with exponential backoff
• Rate limiting: Per-account limits (minute/hour/day) shared across instances
• Priority queuing: Four levels (immediate, high, medium, low) with FIFO within each
• Delivery reports: Results are posted back to your application via HTTP callback
• Multi-tenancy: Multiple organizations can share one instance with separate accounts
• Large file handling: Auto-upload attachments to S3/GCS/Azure and replace with download links
• Connection pooling: SMTP connections are pooled with acquire/release semantics

┌─────────────┐      REST       ┌──────────────────┐      SMTP      ┌─────────────┐
│ Application │ ──────────────► │ genro-mail-proxy │ ─────────────► │ SMTP Server │
└─────────────┘                 └──────────────────┘                └─────────────┘
       ▲                               │
       │                               │
       └───────────────────────────────┘
                delivery reports

When to use it

Consider this proxy when:

• Multiple application instances need shared rate limits for outbound email
• Email delivery should not block your application's main request flow
• Delivery tracking is needed with central logging and Prometheus metrics
• Retry logic is required without implementing it in every service
• Multi-tenant isolation is needed for different organizations or environments

When NOT to use it

This proxy adds operational complexity. Direct SMTP may be simpler when:

• You have a single application instance with low email volume
• Latency is acceptable (direct SMTP adds ~500-600ms per send)
• No retry logic is needed (transactional emails with immediate feedback)
• No rate limiting is required by your SMTP provider
• You prefer fewer moving parts in your infrastructure

Quick start

Docker:

docker run -p 8000:8000 \
  -e GMP_API_TOKEN=your-secret-token \
  genro-mail-proxy

CLI:

pip install genro-mail-proxy
mail-proxy start myserver

Then configure a tenant, add an SMTP account, and start sending messages.

Command-line interface

The mail-proxy CLI manages instances without going through the HTTP API:

# Instance management
mail-proxy list                          # List all instances
mail-proxy start myserver                # Start an instance
mail-proxy stop myserver                 # Stop an instance
mail-proxy myserver info                 # Show instance details

# Tenant management
mail-proxy myserver tenants list         # List tenants
mail-proxy myserver tenants add acme     # Add a tenant (interactive)

# Account management (per tenant)
mail-proxy myserver acme accounts list   # List SMTP accounts
mail-proxy myserver acme accounts add    # Add account (interactive)

# Message operations
mail-proxy myserver acme messages list   # List queued messages
mail-proxy myserver acme send email.eml  # Send from .eml file
mail-proxy myserver acme run-now         # Trigger immediate dispatch

Each instance stores its configuration in ~/.mail-proxy/<name>/mail_service.db. The CLI supports both command-line arguments and interactive prompts for complex operations.

REST API

The proxy exposes a FastAPI REST API secured by X-API-Token:

• POST /commands/add-messages - Queue messages for delivery
• GET /messages - List queued messages
• POST /commands/run-now - Trigger immediate dispatch cycle
• GET /accounts - List SMTP accounts
• GET /metrics - Prometheus metrics

See API Reference for details.

Attachment handling

The proxy supports multiple attachment sources via explicit fetch_mode:

fetch_mode	storage_path example	Description
base64	SGVsbG8gV29ybGQ=	Inline base64-encoded content
filesystem	/tmp/file.pdf	Local filesystem path
endpoint	doc_id=123	HTTP POST to tenant's attachment endpoint
http_url	https://storage.example.com/file.pdf	HTTP GET from external URL

A two-tiered cache (memory + disk) reduces redundant fetches. Filenames can include an MD5 hash marker (report_{MD5:abc123}.pdf) for cache lookup.

Large file offloading

For attachments exceeding a size threshold, the proxy can upload them to external storage (S3, GCS, Azure, or local filesystem) and replace them with download links in the email body.

pip install genro-mail-proxy[large-files]

Configure per-tenant via large_file_config:

{
  "enabled": true,
  "max_size_mb": 10,
  "storage_url": "s3://bucket/mail-attachments",
  "action": "rewrite"
}

Actions: warn (log only), reject (fail message), rewrite (upload and replace with link).

Configuration

Configuration is managed via the CLI. Each instance stores its settings in ~/.mail-proxy/<name>/mail_service.db.

# Start an instance (creates it if new)
mail-proxy start myserver

# Add a tenant with attachment endpoint
mail-proxy myserver tenants add
# Interactive prompts: tenant_id, name, base_url, attachment_path, auth method

# Add an SMTP account
mail-proxy myserver acme accounts add
# Interactive prompts: account_id, host, port, user, password, TLS, rate limits

For Docker deployments, use environment variables (prefixed with GMP_):

docker run -p 8000:8000 \
  -e GMP_API_TOKEN=your-secret-token \
  -e GMP_DB_PATH=/data/mail_service.db \
  -v mail-data:/data \
  genro-mail-proxy

See Usage for all options.

Performance notes

• Request latency: ~30ms to queue a message (vs ~600ms for direct SMTP)
• Throughput: Limited by SMTP provider rate limits, not the proxy
• Memory: Attachment content is held in memory during send; use HTTP endpoints for large files

The SQLite database handles typical workloads but doesn't scale well under high concurrency. For high-volume deployments, use PostgreSQL:

pip install genro-mail-proxy[postgresql]

Then configure the DSN via environment variable or CLI.

Development

pip install -e ".[dev]"
pytest

License

Apache License 2.0 — see LICENSE for details.

Copyright 2025 Softwell S.r.l. — Genropy Team