Go smtp发送邮件，带附件

package main

import (
        "net/smtp"
        "bytes"
        "time"
        "io/ioutil"
        "encoding/base64"
        "strings"
        "log"
)

// define email interface, and implemented auth and send method
type Mail interface {
        Auth()
        Send(message Message) error
}

type SendMail struct {
        user     string
        password string
        host     string
        port     string
        auth     smtp.Auth
}

type Attachment struct {
        name        string
        contentType string
        withFile    bool
}

type Message struct {
        from        string
        to          []string
        cc          []string
        bcc         []string
        subject     string
        body        string
        contentType string
        attachment  Attachment
}

func main() {
        var mail Mail
        mail = &SendMail{user: "chunyunzeng@hotmail.com", password: "password", host: "smtp.mxhichina.com", port: "25"}
        message := Message{from: "chunyunzeng@hotmail.com",
                to: []string{"850808158@qq.com"},
                cc: []string{},
                bcc: []string{},
                subject: "HELLO WORLD",
                body: "",
                contentType: "text/plain;charset=utf-8",
                attachment: Attachment{
                        name:        "test.jpg",
                        contentType: "image/jpg",
                        withFile:    true,
                },
        }
        mail.Send(message)
}

func (mail *SendMail) Auth() {
        mail.auth = smtp.PlainAuth("", mail.user, mail.password, mail.host)
}

func (mail SendMail) Send(message Message) error {
        mail.Auth()
        buffer := bytes.NewBuffer(nil)
        boundary := "GoBoundary"
        Header := make(map[string]string)
        Header["From"] = message.from
        Header["To"] = strings.Join(message.to, ";")
        Header["Cc"] = strings.Join(message.cc, ";")
        Header["Bcc"] = strings.Join(message.bcc, ";")
        Header["Subject"] = message.subject
        Header["Content-Type"] = "multipart/mixed;boundary=" + boundary
        Header["Mime-Version"] = "1.0"
        Header["Date"] = time.Now().String()
        mail.writeHeader(buffer, Header)

        body := "\r\n--" + boundary + "\r\n"
        body += "Content-Type:" + message.contentType + "\r\n"
        body += "\r\n" + message.body + "\r\n"
        buffer.WriteString(body)

        if message.attachment.withFile {
                attachment := "\r\n--" + boundary + "\r\n"
                attachment += "Content-Transfer-Encoding:base64\r\n"
                attachment += "Content-Disposition:attachment\r\n"
                attachment += "Content-Type:" + message.attachment.contentType + ";name=\"" + message.attachment.name + "\"\r\n"
                buffer.WriteString(attachment)
                defer func() {
                        if err := recover(); err != nil {
                                log.Fatalln(err)
                        }
                }()
                mail.writeFile(buffer, message.attachment.name)
        }

        buffer.WriteString("\r\n--" + boundary + "--")
        smtp.SendMail(mail.host+":"+mail.port, mail.auth, message.from, message.to, buffer.Bytes())
        return nil
}

func (mail SendMail) writeHeader(buffer *bytes.Buffer, Header map[string]string) string {
        header := ""
        for key, value := range Header {
                header += key + ":" + value + "\r\n"
        }
        header += "\r\n"
        buffer.WriteString(header)
        return header
}

// read and write the file to buffer
func (mail SendMail) writeFile(buffer *bytes.Buffer, fileName string) {
        file, err := ioutil.ReadFile(fileName)
        if err != nil {
                panic(err.Error())
        }
        payload := make([]byte, base64.StdEncoding.EncodedLen(len(file)))
        base64.StdEncoding.Encode(payload, file)
        buffer.WriteString("\r\n")
        for index, line := 0, len(payload); index < line; index++ {
                buffer.WriteByte(payload[index])
                if (index+1)%76 == 0 {
                        buffer.WriteString("\r\n")
                }
        }
}