go-milter

A Go library to write mail filters.

Features

• With this library you can write both the client (MTA/SMTP-Server) and server (milter filter) in pure Go without sendmail's libmilter.
• Easy wrapper of the milter protocol that abstracts away many milter protocol quirks and lets you write mail filters with little effort.
• UTF-8 support
• IDNA support
• Client & Server support milter protocol version 6 with all features. E.g.:
  • all milter events including DATA, UNKNOWN, ABORT and QUIT NEW CONNECTION
  • milter can skip e.g. body chunks when it does not need all chunks
  • milter can send progress notifications when response can take some time
  • milter can automatically instruct the MTA which macros it needs.
• Automatic integration tests that test the compatibility with Postfix and Sendmail.

Installation

go get -u github.com/d--j/go-milter

Usage

The following example is a milter filter that adds [⚠️EXTERNAL] to the subject of all messages of unauthenticated users.

See GoDoc for more documentation and an example for a milter client or a raw milter server.

package main

import (
  "context"
  "flag"
  "log"
  "strings"

  "github.com/d--j/go-milter/mailfilter"
)

func main() {
  // parse commandline arguments
  var protocol, address string
  flag.StringVar(&protocol, "proto", "tcp", "Protocol family (unix or tcp)")
  flag.StringVar(&address, "addr", "127.0.0.1:10003", "Bind to address or unix domain socket")
  flag.Parse()

  // create and start the mail filter
  mailFilter, err := mailfilter.New(protocol, address,
    func(_ context.Context, trx mailfilter.Trx) (mailfilter.Decision, error) {
      // Quarantine mail when it is addressed to our SPAM trap
      if trx.HasRcptTo("spam-trap@スパム.example.com") {
        return mailfilter.QuarantineResponse("train as spam"), nil
      }
      // Prefix subject with [⚠️EXTERNAL] when user is not logged in
      if trx.MailFrom().AuthenticatedUser() == "" {
        subject, _ := trx.Headers().Subject()
        if !strings.HasPrefix(subject, "[⚠️EXTERNAL] ") {
          subject = "[⚠️EXTERNAL] " + subject
          trx.Headers().SetSubject(subject)
        }
      }
      return mailfilter.Accept, nil
    },
    // optimization: call the decision function when all headers were sent to us
    mailfilter.WithDecisionAt(mailfilter.DecisionAtEndOfHeaders),
  )
  if err != nil {
    log.Fatal(err)
  }
  log.Printf("Started milter on %s:%s", mailFilter.Addr().Network(), mailFilter.Addr().String())

  // wait for the mail filter to end
  mailFilter.Wait()
}

License

BSD 2-Clause

Credits

Based on https://github.com/emersion/go-milter by Simon Ser which is based on https://github.com/phalaaxx/milter by Bozhin Zafirov. Max Mazurov made major contributions to this code as well.