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)
}

Test your SMTP connection to make sure your HOST/ISP isn't blocking SMTP

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

Simple example of doing the SMTP dance yourself.

https://play.golang.org/p/CzsSxXfnT_F

https://play.golang.org/p/D3nzBtHXk-0

Recently stumbled across this and want to point out you can use a net.Resolver to set the context for proper timeouts.

func LookupMX(ctx context.Context, domain string) ([]*net.MX, error) {
	var r net.Resolver
	return r.LookupMX(ctx, domain)
}

Wondering: in composeMimeMail you pass subject to encodeRFC2047 which interprets its arg as an email address. What's the idea?