A Go SMTP Server

Introduction

I wanted a way to bring email as a first class citizen in go. I modeled the api much like the http package. There were a few translation errors that I'm still trying to work out, things that are harder due to the fact that smtp is a stateful protocol. I'd welcome feedback on this.

A Very simple server

The smtp package works by offering a muxer that will route incoming email to registered handlers. A simple server would register an 'all' handler, and then listen on the smtp port.

import "github.com/murphysean/smtp"

smtp.HandleFunc("*@*", func(envelope *smtp.Envelope) error {
	fmt.Println("Message Received", envelope.MessageTo)
	fmt.Println("From:", envelope.MessageFrom, envelope.RemoteAddr)
	fmt.Println("To:", envelope.MessageTo)
	fn := "emails/" + time.Now().Format(time.RFC3339) + ".eml"
	ioutil.WriteFile(fn, b, os.ModePerm)
	fmt.Println("Wrote to " + fn)
	return nil
}

log.Fatal(smtp.ListenAndServe(":smtp", nil))

Now all incoming messages will be logged and saved to the emails directory.

The Handler pattern has two parts, the local and the domain. The matching algorithm:

1. Start with the domain
2. If there is a match 1. Move to step 2
3. If there is not a match 1. Start at step 1 with domain = "*"
4. Check the local portion
5. If there is a match 1. Move to step 3
6. If there is not a match 1. Retry step 2 with local = "*"
7. Call the registered handler

If there is no registered handler the server will return an error to the client and disregard the email.

Additional Options

Security

The smtp server also supports the STARTTLS option, if you use the ListenAndServeTLS variant. You can also further customize the tls config as well.

server := smtp.Server{Name: "example.com", Debug: true}
config := &tls.Config{MinVersion:tls.VersionSSL30}
server.TLSConfig = config
log.Fatal(server.ListenAndServeTLS(":smtp", "cert.pem", "key.pem", nil))

Naming and Debugging

As shown in the previous snippet you can also give your server a name (default = localhost). Naming lends credibility to your server, that some clients seem to require.

Debugging is pretty verbose and dumps the entire protocol out to stderr. It is really handy for troubleshooting particularly annoying clients.

Authentication

The smtp server also supports authentication via the PLAIN method. Ideally this would be coupled with STARTTLS to ensure secrecy of passwords in transit. You can do this by creating a custom server and registering the AUTH callback. This will be called everytime someone attempts to authenticate.

server.Auth = func(username, password, remoteAddress string) error {
	if username == "user" && password == "p@$$w0rd" {
		return nil
	}
	return errors.New("Nope!")
}

Addressing and preventing open-relay

Since your callback is only called once the smtp protocol has progressed to the data point, meaning the sender and recipient have been specified, the server also offers an Addressable callback that can be used to deny unknown recipients.

server.Addressable = func(user, address string) bool {
	if user != ""{
		//Allow relay for authenticated users
		return true
	}
	if strings.HasSuffix(address, "example.com"){
		return true
	}
	return false
}