The tiny mail command write in golang.

mail.go

package main

import (
	"crypto/tls"
	"flag"
	"fmt"
	"io/ioutil"
	"net"
	"net/smtp"
	"os"
	"os/user"
	"strings"
)

const sslPort = 465

func main() {
	subject := flag.String("subject", "", "subject")

	flag.Parse()

	address := flag.Arg(0)
	if address == "" {
		panic("Invalid email address")
	}

	msg := getMessage(*subject)

	err := sendMail(address, msg)
	if err != nil {
		panic(err)
	}
}

func sendMail(to string, msg []byte) error {
	mailname := getMailname()
	from := fmt.Sprintf("%s@%s", getUsername(), mailname)

	server, conn := getServerConn(to)

	c, err := smtp.NewClient(conn, server)
	if err != nil {
		return err
	}

	defer c.Close()

	if err = c.Hello(mailname); err != nil {
		return err
	}

	if err = c.Mail(from); err != nil {
		return err
	}
	if err = c.Rcpt(to); err != nil {
		return err
	}

	w, err := c.Data()
	if err != nil {
		return err
	}

	_, err = w.Write(msg)
	if err != nil {
		return err
	}
	err = w.Close()
	if err != nil {
		return err
	}

	return c.Quit()
}

func getUsername() string {
	user, err := user.Current()
	if err != nil {
		panic(err)
	}

	return user.Username
}

func getMailname() string {
	data, err := ioutil.ReadFile("/etc/mailname")
	if err != nil {
		hostname, err := os.Hostname()
		if err != nil {
			panic(err)
		}

		return hostname
	}

	content := string(data)
	return strings.TrimSpace(strings.SplitN(content, "\n", 1)[0])
}

func getServerConn(address string) (string, *tls.Conn) {
	server := strings.Split(address, "@")[1]
	mxs, err := net.LookupMX(server)
	if err != nil {
		panic(err)
	}

	host := mxs[0].Host

	tlsConfig := &tls.Config{
		ServerName: server,
	}

	conn, err := tls.Dial("tcp", fmt.Sprintf("%s:%d", host, sslPort), tlsConfig)
	if err != nil {
		panic(err)
	}

	return server, conn
}

func getMessage(subject string) []byte {
	buf, err := ioutil.ReadAll(os.Stdin)
	if err != nil {
		panic(err)
	}

	if subject == "" {
		return buf
	}

	h := fmt.Sprintf("Subject: %s\r\n\r\n", subject)
	buf = append([]byte(h), buf...)
	return buf
}