Simple SMTP mail sender in golang that can send email directly to anyone. Super basic, but great for sending notice emails. Upgrades to STARTTLS if it can.

mail.go

package main

import (
	"encoding/base64"
	"fmt"
	"log"
	"net"
	"net/mail"
	"net/smtp"
	"strings"
	"time"
)

// telnet gmail-smtp-in.l.google.com 25
// openssl s_client -connect gmail-smtp-in.l.google.com:25

func main() {

	var (
		mx string
		err error
	)
	
	// Connect to the server, authenticate, set the sender and recipient,
	// and send the email all in one step.
	from := "me@example.com"
	to := "bob@gmail.com"
	subject := "Test Email"
	body := "This is the email body at " + time.Now().String()

	msg := composeMimeMail(to, from, subject, body)

	mx, err = getMXRecord(to)
	if err != nil {
		log.Fatal(err)
	}

	err = smtp.SendMail(mx+":25", nil, from, []string{to}, msg)
	if err != nil {
		log.Fatal(err)
	}
}

func getMXRecord(to string) (mx string, err error) {
	var e *mail.Address
	e, err = mail.ParseAddress(to)
	if err != nil {
		return
	}

	domain := strings.Split(e.Address, "@")[1]

	var mxs []*net.MX
	mxs, err = net.LookupMX(domain)

	if err != nil {
		return
	}

	for _, x := range mxs {
		mx = x.Host
		return
	}

	return
}

// Never fails, tries to format the address if possible
func formatEmailAddress(addr string) string {
	e, err := mail.ParseAddress(addr)
	if err != nil {
		return addr
	}
	return e.String()
}

func encodeRFC2047(str string) string {
	// use mail's rfc2047 to encode any string
	addr := mail.Address{Address: str}
	return strings.Trim(addr.String(), " <>")
}

func composeMimeMail(to string, from string, subject string, body string) []byte {
	header := make(map[string]string)
	header["From"] = formatEmailAddress(from)
	header["To"] = formatEmailAddress(to)
	header["Subject"] = encodeRFC2047(subject)
	header["MIME-Version"] = "1.0"
	header["Content-Type"] = "text/plain; charset=\"utf-8\""
	header["Content-Transfer-Encoding"] = "base64"

	message := ""
	for k, v := range header {
		message += fmt.Sprintf("%s: %s\r\n", k, v)
	}
	message += "\r\n" + base64.StdEncoding.EncodeToString([]byte(body))

	return []byte(message)
}