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: "[email protected]", password: "password", host: "smtp.mxhichina.com", port: "25"}
    message := Message{from: "[email protected]",
        to: []string{"[email protected]"},
        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")
        }
    }
}