smtprelay

This is a simple SMTP relay, that accepts mail via SMTP and forwards it to another SMTP server. Its main purpose is focused on observability, with metrics exposed in the Prometheus format, and structured logging.

Grafana Labs uses this internally, and as such it is not intended to be a generally-supported product. In particular, we reserve the right to make breaking changes, remove functionality, or otherwise change the code without notice.

We are happy to consider issues and pull requests.

Development

• make build - build go code
• make docker build docker image
• make docker-tag - build and push docker image
• make docker-push - build, tag, and push docker image

Update DOCKER_IMAGE in Makefile to change Docker Repo.

Deployment

Configuration

There are two ways to provide configuration

1. .ini file (see smtprelay.ini for example)

   $ ./smtprelay -config=smtprelay.ini

2. command line arguments for each config option.

   $ ./smtprelay -listen=127.0.0.1:2525 -hostname=localhost -remote_host=smtp.example.com:587 -remote_user=noreply@example.com

You can mix and match, see priority to see which config value will be used

config priority - source

1. use value set via command-line,
2. if not set, use value from ini file,
3. at last, use default value.

NOTE: If remote_pass is not set at the end, It will try to read it from REMOTE_PASS environment variable.

It was added to support loading the secret from an environment variable.

Use ./smtprelay -help for help on config options.

Metrics

Prometheus metrics are available at <url>:8080/metrics.

The listening address can be changed by setting metrics_listen.

Logs

Structured logs are written to stderr.

The log level is INFO by default, and can be changed by setting log_level.

Tracing

Tracing is done using OpenTelemetry. Only OTLP over gRPC is supported. The exporter can be configured with environment variables, such as OTEL_EXPORTER_OTLP_TRACES_ENDPOINT. Sampling can be adjusted dynamically using the Jaeger sampler manager API. Set the sampling server URL with JAEGER_SAMPLER_MANAGER_HOST_PORT.

To disable tracing, set OTEL_SDK_DISABLED to "true".

Trace propagation uses the W3C Trace Context format, using email MIME headers as carriers. The header are assumed to be traceparent and tracestate.

Note that only the relay's handling of the message itself is traced (after the DATA command), not the rest of the SMTP conversation. For this reason, the span's timing will miss the time spent before the DATA command.

Docker

We publish images on DockerHub at grafana/smtprelay

Manual Testing

To test code or config, start smtprelay, and send test email using swaks.

Tip: you can install swaks using sudo apt install swaks on Ubuntu.

$ swaks --to=test@example.com --from=noreply@example.com --server=localhost:2525 --h-Subject="Hello from smtprelay" --body="This is test email from smtprelay"

To test with trace propagation, start smtprelay using air, and use otel-cli:

$ otel-cli exec -s swaks -n "send e-mail" -- sh -c 'swaks --to alice@example.com --from=bob@example.com --server localhost:2525 --h-Subject: "Hello from smtprelay" -h-Traceparent: "${TRACEPARENT}" --body "This is a test email from smtprelay"'

Acknowledgements

This started as a fork of github.com/decke/smtprelay. We thank the original authors for their work.