MQTT to Email Gateway Service

This Go service acts as a bridge between an MQTT message broker and an SMTP server. It listens for messages on a specified MQTT topic, parses them as email requests, and sends emails accordingly. It features configurable rate limiting, SMTP options, and structured logging.

Features

• MQTT Driven: Consumes email requests from an MQTT topic.
• SMTP Email Sending: Sends emails using a configurable SMTP server.
• Flexible Configuration: Easily configurable via environment variables or a .env file.
• Rate Limiting:
  • Supports email sending rate limits per minute (primary).
  • Discard Behavior: If the rate limit is exceeded, new email requests are discarded (not queued or waited upon).
• SMTP TLS Control: Supports TLSMandatory, TLSOpportunistic, and NoTLS policies for SMTP connections, along with options to skip TLS certificate verification for development/testing.
• Input Validation: Validates incoming MQTT message payloads and email recipient formats.
• JSON Message Body Formatting: Attempts to pretty-print JSON strings if provided as the message body.
• Structured Logging: Outputs logs in JSON format for easier parsing and analysis.
• Graceful Shutdown: Handles SIGINT and SIGTERM for clean shutdown.
• MQTT Auto-Reconnect: Automatically attempts to reconnect to the MQTT broker if the connection is lost.

Configuration

The service is configured using environment variables. You can also place these variables in a .env file in the root directory of the application.

Key Environment Variables:

SMTP Configuration:

• SMTP_HOST: (Required) Hostname or IP address of your SMTP server.
• SMTP_PORT: Port number for the SMTP server (default: 587).
• SMTP_USERNAME: Username for SMTP authentication.
• SMTP_PASSWORD: Password for SMTP authentication.
• SMTP_TLS_SKIP_VERIFY: Set to true to skip TLS certificate verification (e.g., for self-signed certificates in dev). Default: false.
• SMTP_FROM: The "From" email address for outgoing emails. If not set, SMTP_USERNAME is used.
• SMTP_FROM_NAME: The display name for the "From" address (e.g., "My Application").
• SMTP_TIMEOUT: Timeout in seconds for SMTP connection and operations (default: 15).
• SMTP_TLS_POLICY: TLS policy for SMTP. Options:
  • TLSMandatory (default): Always use TLS.
  • TLSOpportunistic: Use TLS if available (STARTTLS).
  • NoTLS: Do not use TLS.

MQTT Configuration:

• MQTT_BROKER: URL of the MQTT broker (default: tcp://localhost:1883).
• MQTT_TOPIC: MQTT topic to subscribe to for email requests (default: email/send).
• MQTT_CLIENT_ID: Client ID for the MQTT connection (default: mqtt-mail-gateway).
• MQTT_QOS: MQTT Quality of Service level for subscription (0, 1, or 2) (default: 1).
• MQTT_USERNAME: Username for MQTT broker authentication.
• MQTT_PASSWORD: Password for MQTT broker authentication.

Rate Limiting Configuration:

• SMTP_RATE_LIMIT_PER_MINUTE: Maximum emails to send per minute. If this is set to a positive value, it takes precedence. If the limit is exceeded, emails are discarded. (Default: 0, meaning per-second configuration is checked).
  • Example: SMTP_RATE_LIMIT_PER_MINUTE=60 (allows up to 60 emails per minute, then discards further requests until capacity is available again).

Usage

Publish a JSON message to the configured MQTT_TOPIC with the following structure:

{
  "recipients": "user1@example.com, user2@example.com; user3@example.com",
  "subject": "Hello from MQTT!",
  "message": "This is the body of your email. It can be plain text or a JSON string to be pretty-printed."
}