Simple Golang SMTP relay/proxy server

Topics
#golang #proxy-server #smtp-server #smtp-relay #ssl #load-balancer

Bernhard Froehlich c08ff67b9e Bump Go version for CI builds to 1.16 4 éve
.github 7b199cc6ae Bump Go version for CI builds to 1.16 4 éve
cmd 009ae8f73a hasher: Check number of arguments 4 éve
LICENSE c707438ae8 Update MIT license template 6 éve
README.md 2afbe67407 Add Go Report Card badge 5 éve
auth.go 7f34fcbc99 gofmt: Fix formatting 4 éve
auth_test.go 7f34fcbc99 gofmt: Fix formatting 4 éve
config.go 247d187576 Don't allow a configuration requiring auth with a non-TLS listener 4 éve
config_test.go ca1ccd85e3 Refactor parsing of -listen string out into separate config function 4 éve
go.mod 49c6880175 Bump github.com/chrj/smtpd from 0.2.0 to 0.3.0 4 éve
go.sum 49c6880175 Bump github.com/chrj/smtpd from 0.2.0 to 0.3.0 4 éve
logger.go 9921b38046 Explicitly configure default logfile for stderr 4 éve
main.go 45a676e274 Add localAuthRequired() helper function 4 éve
main_test.go 7f34fcbc99 gofmt: Fix formatting 4 éve
smtp.go b36ed8eddb net/smtp: adds support for the SMTPUTF8 extension 4 éve
smtprelay.ini 22ef0c2ee6 Move SMTP auth setup to ConfigLoad() 4 éve

README.md

smtprelay

Go Report Card

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