sendmail with tls

mail.go

package main

import (
	"crypto/tls"
	"errors"
	"log"
	"net"
	"net/smtp"
)

func main() {
	// Set up authentication information.
	auth := smtp.PlainAuth("", "[email protected]", "password", "mail.example.com")

	// Connect to the server, authenticate, set the sender and recipient,
	// and send the email all in one step.
	to := []string{"[email protected]"}
	msg := []byte("To: [email protected]\r\n" +
		"Subject: discount Gophers!\r\n" +
		"\r\n" +
		"This is the email body.\r\n")
	// directly use tls port 465
	err := smtp.SendMail("mail.example.com:465", auth, "[email protected]", to, msg)
	if err != nil {
		log.Fatal(err)
	}
}

// SendMail 用于发送邮件。和标准库中不同的是默认使用TLS，而不是先用普通连接之后再升级
// 邮件提供商提供的smtp发送端口都要直接用TLS连接的，使用标准库会返回错误EOF
// 如果使用标准库需要用25端口，而不是帮助中说的465或995端口
func SendMail(addr string, a smtp.Auth, from string, to []string, msg []byte) error {
	hostname, _, err := net.SplitHostPort(addr)
	if err != nil {
		return err
	}
	conn, err := tls.Dial("tcp", addr, &tls.Config{ServerName: hostname})
	if err != nil {
		return err
	}
	c, err := smtp.NewClient(conn, hostname)
	if err != nil {
		return err
	}
	defer c.Close()
	if err = c.Auth(a); err != nil {
		return err
	}
	if err = c.Mail(from); err != nil {
		return err
	}
	for _, addr := range to {
		if err = c.Rcpt(addr); 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()
}

type plainAuth struct {
	identity, username, password string
	host                         string
}

func (a *plainAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
	if server.Name != a.host {
		return "", nil, errors.New("wrong host name")
	}
	resp := []byte(a.identity + "\x00" + a.username + "\x00" + a.password)
	return "PLAIN", resp, nil
}

func (a *plainAuth) Next(fromServer []byte, more bool) ([]byte, error) {
	if more {
		// We've already sent everything.
		return nil, errors.New("unexpected server challenge")
	}
	return nil, nil
}