smtpd

An SMTP server package written in Go, in the style of the built-in HTTP server. It meets the minimum requirements specified by RFC 2821 & 5321.

It is based on Brad Fitzpatrick's go-smtpd. The differences can be summarised as:

A simplified message handler
Changes made for RFC compliance
Testing has been added
Code refactoring

Features

A single message handler for simple mail handling with native data types.
RFC compliance. It implements the minimum command set, responds to commands and adds a valid Received header to messages as specified in RFC 2821 & 5321.
Customisable listening address and port. It defaults to listening on all addresses on port 25 if unset.
Customisable host name and application name. It defaults to the system hostname and "smtpd" application name if they are unset.

Usage

In general: create the server and pass a handler function to it as for the HTTP server. The server function has the following definition:

func ListenAndServe(addr string, handler Handler, appname string, hostname string) error

The handler function must have the following definition:

func handler(remoteAddr net.Addr, from string, to []string, data []byte)

The parameters are:

remoteAddr: remote end of the TCP connection i.e. the mail client's IP address and port.
from: the email address sent by the client in the MAIL command.
to: the set of email addresses sent by the client in the RCPT command.
data: the raw bytes of the mail message.

Example

The following example code creates a new server with the name "MyServerApp" that listens on the localhost address and port 2525. Upon receipt of a new mail message, the handler function parses the mail and prints the subject header.

package main

import (
	"bytes"
	"log"
	"net"
	"net/mail"

	"github.com/mhale/smtpd"
)

func mailHandler(origin net.Addr, from string, to []string, data []byte) {
	msg, _ := mail.ReadMessage(bytes.NewReader(data))
	subject := msg.Header.Get("Subject")
	log.Printf("Received mail from %s for %s with subject %s", from, to[0], subject)
}

func main() {
	smtpd.ListenAndServe("127.0.0.1:2525", mailHandler, "MyServerApp", "")
}

Testing

The tests cover the supported SMTP command set and line parsing. A single server is created listening on an ephemeral port (52525) for the duration of the tests. Each test creates a new client connection for processing commands.

Licensing