Configuration - MailLaser Docs

MailLaser is configured entirely through environment variables. Set them directly in your shell, pass them via Docker's -e flag, or place them in a .env file in the working directory.

Required variables

These must be set for MailLaser to start. If either is missing, the application exits with an error.

Variable	Description
`MAIL_LASER_TARGET_EMAILS`	Comma-separated list of email addresses to accept. At least one address is required. Whitespace around commas is trimmed.
`MAIL_LASER_WEBHOOK_URL`	The URL where email payloads are forwarded via HTTP POST.

Optional variables

These have sensible defaults and can be left unset for most deployments.

Server settings

Variable	Default	Description
`MAIL_LASER_BIND_ADDRESS`	`0.0.0.0`	IP address the SMTP server binds to.
`MAIL_LASER_PORT`	`2525`	Port the SMTP server listens on. Must be a valid port number (1-65535).
`MAIL_LASER_HEALTH_BIND_ADDRESS`	`0.0.0.0`	IP address the health check server binds to.
`MAIL_LASER_HEALTH_PORT`	`8080`	Port the health check server listens on.

Webhook settings

Variable	Default	Description
`MAIL_LASER_WEBHOOK_TIMEOUT`	`30`	Seconds to wait for a webhook response before timing out.
`MAIL_LASER_WEBHOOK_MAX_RETRIES`	`3`	Maximum retry attempts after a failed webhook delivery.

Circuit breaker settings

Variable	Default	Description
`MAIL_LASER_CIRCUIT_BREAKER_THRESHOLD`	`5`	Consecutive failures before the circuit breaker opens.
`MAIL_LASER_CIRCUIT_BREAKER_RESET`	`60`	Seconds before an open circuit breaker transitions to half-open.

Header passthrough

Variable	Default	Description
`MAIL_LASER_HEADER_PREFIX`	(none)	Comma-separated list of header name prefixes to forward. Case-insensitive matching. See Header passthrough.

Logging

Variable	Default	Description
`RUST_LOG`	`info`	Controls log verbosity. Common values: `error`, `warn`, `info`, `debug`, `trace`.

You can target specific modules for granular control:

RUST_LOG=mail_laser=debug -- Debug logging for MailLaser only
RUST_LOG=info,mail_laser::smtp=debug -- Info globally, debug for the SMTP module
RUST_LOG=info,mail_laser::webhook=trace -- Info globally, trace for the webhook module

Logs are written to stdout. Use docker logs mail-laser when running in Docker.

Using a .env file

Place a .env file in the same directory where the binary runs. MailLaser loads it automatically at startup using the dotenv crate.

# .env
MAIL_LASER_TARGET_EMAILS=alerts@mydomain.com,support@mydomain.com
MAIL_LASER_WEBHOOK_URL=https://hooks.example.com/services/T000/B001/XXX
MAIL_LASER_PORT=2525
MAIL_LASER_HEALTH_PORT=8080
MAIL_LASER_HEADER_PREFIX=X-Custom,X-My-App
RUST_LOG=info

Precedence

Environment variables set in the shell take precedence over values in the .env file. If you set MAIL_LASER_PORT=3000 in both the shell and the .env file, the shell value wins.

Validation behavior

MailLaser validates configuration at startup:

Missing required variables: The application logs an error and exits immediately.
Empty target emails: If MAIL_LASER_TARGET_EMAILS is set but contains no valid addresses after trimming and splitting, startup fails.
Invalid port numbers: If MAIL_LASER_PORT or MAIL_LASER_HEALTH_PORT cannot be parsed as a valid u16, startup fails with a descriptive error.
Invalid numeric values: Timeout, retry, and circuit breaker settings must be valid integers of their expected types.

All loaded configuration values are logged at info level during startup, making it straightforward to verify what settings are in effect.

Example: minimal configuration

The smallest viable configuration requires only the two required variables:

MAIL_LASER_TARGET_EMAILS="inbox@myapp.com" \
MAIL_LASER_WEBHOOK_URL="https://myapp.com/email-hook" \
./mail_laser

This starts the SMTP server on 0.0.0.0:2525 and the health check on 0.0.0.0:8080 with all default resilience settings.