smtprelay

smtprelay

Simple Golang based SMTP relay/proxy server that accepts mail via SMTP and forwards it directly to another SMTP server.

Why another SMTP server?

Outgoing mails are usually send via SMTP to an MTA (Mail Transfer Agent) which is one of Postfix, Exim, Sendmail or OpenSMTPD on UNIX/Linux in most cases. You really don't want to setup and maintain any of those full blown kitchensinks yourself because they are complex, fragile and hard to configure.

My use case is simple. I need to send automatically generated mails from cron via msmtp/sSMTP/dma, mails from various services and network printers to GMail without giving away my GMail credentials to each device which produces mail.

Main features

• Supports SMTPS/TLS (465), STARTTLS (587) and unencrypted SMTP (25)
• Checks for sender, receiver, client IP
• Authentication support with file (LOGIN, PLAIN)
• Enforce encryption for authentication
• Forwards all mail to a smarthost (GMail, MailGun or any other SMTP server)
• Small codebase
• IPv6 support

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

1. command line arguments for each config option.

./smtprelay -listen=127.0.0.1:2525 -hostname=localhost -remote_host=smtp.mailgun.org:2525 -remote_user=hosted-grafana@grafana.net

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 secret from environment variable

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

Metrics

Service exposes Prometheus metrics on <url>:8080/metrics, metrics port is configurable using metrics_listen

Logs

Structured logs are written to stdout by default, can be configured to write to file by setting logfile to full path of logfile.

log level is debug by default, can be changed by setting log_level.

supported levels: trace, debug, info, warning, error, fatal, panic

Docker

We publish images on DockerHub at grafana/smtprelay

See all tags

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 <email> --from=<email> --server localhost:2525 --h-Subject: "Hello from smtprelay" --h-Body: "This is test email from smtprelay"