SMTP mail transfer sample program for gmail and office365.

mail.go

package main

import (
	"bytes"
	"errors"
	"fmt"
	"html/template"
	"log"
	"net"
	"net/mail"
	"net/smtp"
	"os"
	"strings"
	"time"
)

type plainLoginAuth struct {
	actual                   smtp.Auth
	username, password, host string
}

// PlainLoginAuth creates new plainLoginAuth strcut as smtp.Auth interface.
// plainLoginAuth is a wrapper for handling both PLAIN and LOGIN type authentication according to SMTP servers extention.
func PlainLoginAuth(username, password, host string) smtp.Auth {
	return &plainLoginAuth{username: username, password: password, host: host}
}

func (a *plainLoginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
	for _, mechanism := range server.Auth {
		if mechanism == "PLAIN" {
			a.actual = smtp.PlainAuth("", a.username, a.password, a.host)
			return a.actual.Start(server)
		} else if mechanism == "LOGIN" {
			a.actual = LoginAuth(a.username, a.password)
			return a.actual.Start(server)
		}
	}
	return "", nil, errors.New("smtp server doesn't support both PLAIN type AUTH and LOGIN TYPE AUTH")
}

func (a *plainLoginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
	return a.actual.Next(fromServer, more)
}

type loginAuth struct {
	username, password string
}

// LoginAuth creates new loginAuth strcut as smtp.Auth interface.
// loginAuth handle LOGIN type authentication in SMTP.
func LoginAuth(username, password string) smtp.Auth {
	return &loginAuth{username, password}
}

func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
	if !server.TLS {
		advertised := false
		for _, mechanism := range server.Auth {
			if mechanism == "LOGIN" {
				advertised = true
				break
			}
		}
		if !advertised {
			return "", nil, errors.New("unencrypted connection")
		}
	}
	return "LOGIN", []byte{}, nil
}

func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
	if more {
		switch string(fromServer) {
		case "Username:":
			return []byte(a.username), nil
		case "Password:":
			return []byte(a.password), nil
		default:
			return nil, fmt.Errorf("unkown message from server %s", fromServer)
		}
	}
	return nil, nil
}

const (
	exitSuccsess int = iota // 0
	exitError    int = iota // 1
)

func run() int {
	// set your mail server and login information into environment variables.
	// SMTP_SERVER variable accespts format smtp_server_host:port style.
	smtpSvr := os.Getenv("SMTP_SERVER")
	smtpHost, _, err := net.SplitHostPort(smtpSvr)
	if err != nil {
		log.Println("please set environment variable for SMTP_SERVER properly. got ", smtpSvr, " ", err)
		return exitError
	}

	// set authentication information into environment variables also.
	smtpUser := os.Getenv("SMTP_USER")
	smtpPass := os.Getenv("SMTP_PASS")
	if smtpUser == "" || smtpPass == "" {
		log.Println("please set environment variable for SMTP_USER and SMTP_PASS")
		return exitError
	}

	// create mail body from mail template.
	// this template have to follow RFC822 style, although the \n newline code will be converted to \r\n.
	// after executing mail template, the mail will be parsed to obtain its header contents.
	tmpl, err := template.New("mail").Parse(`From: me <me@example.com>
To: my manager <to@example.com>
Subject: PLEASE DO THIS RIGHT NOW

PLEASE DO FOLLWOING ACTION
{{range .TaskList}} - DO {{.}} 
{{end}}
`)
	if err != nil {
		log.Println(err)
		return exitError
	}

	t := struct{ TaskList []string }{[]string{"TASK1", "TASK2"}}
	var mailBody string
	buf := bytes.NewBufferString(mailBody)
	if err := tmpl.Execute(buf, t); err != nil {
		log.Println(err)
		return exitError
	}

	r := strings.NewReader(buf.String())
	m, err := mail.ReadMessage(r)
	if err != nil {
		log.Println(err)
		return exitError
	}

	fromList, err := m.Header.AddressList("From")
	if err != nil {
		log.Println(err)
		return exitError
	}
	// from have to be 1.
	from := fromList[0].Address

	var to []string
	toList, err := m.Header.AddressList("To")
	if err != nil {
		log.Println(err)
		return exitError
	}

	for _, addr := range toList {
		to = append(to, addr.Address)
	}

	msg := "Date: " + time.Now().Format(time.RFC1123Z) + "\r\n" + buf.String()
	// take care the case when msg contains mixed newline char.
	msg = strings.Replace(msg, "\r\n", "\n", -1)
	msg = strings.Replace(msg, "\n", "\r\n", -1)

	// this smtp authentication mechanism can proceed both PLAIN type and LOGIN type authentication.
	auth := PlainLoginAuth(smtpUser, smtpPass, smtpHost)

	if err := smtp.SendMail(smtpSvr, auth, from, to, []byte(msg)); err != nil {
		log.Println(err)
		return exitError
	}

	return exitSuccsess
}

func main() {
	os.Exit(run())
}