依据 smtp协议的简单golang 的发邮件实现

--THIS_IS_BOUNDARY_JUST_MAKE_YOURS //boundary前边需要加上连接符 -- ， 首部和第一个boundary之间有两个空行
Content-Type:text/plain;chart-set=utf-8
//单个部分的首部和正文间有个空行
这是正文1
这是正文2

--THIS_IS_BOUNDARY_JUST_MAKE_YOURS //每个部分的与上一部分之间一个空行
Content-Type：image/jpg;name="test.jpg"
Content-Transfer-Encoding:base64
Content-Description:这个是描述
//单个部分的首部和正文间有个空行
base64编码的文件 //文件内容使用base64 编码，单行不超过80字节，需要插入\r\n进行换行
--THIS_IS_BOUNDARY_JUST_MAKE_YOURS-- //最后结束的标识--boundary--
golang 代码实现如下

//email/email.go

package email

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

//SendEmailWithAttachment : send email with attachment
func SendEmailWithAttachment(user, passwd, host, to, subject string) error {
hp := strings.Split(host, ":")
auth := smtp.PlainAuth("", user, passwd, hp[0])
buffer := bytes.NewBuffer(nil)

boudary := "THIS_IS_BOUNDARY_JUST_MAKE_YOURS"
header := fmt.Sprintf("To:%s\r\n"+
    "From:%s\r\n"+
    "Subject:%s\r\n"+
    "Content-Type:multipart/mixed;Boundary=\"%s\"\r\n"+
    "Mime-Version:1.0\r\n"+
    "Date:%s\r\n", to, user, subject, boudary, time.Now().String())
buffer.WriteString(header)
fmt.Print(header)

msg1 := "\r\n\r\n--" + boudary + "\r\n" + "Content-Type:text/plain;charset=utf-8\r\n\r\n这是正文啊\r\n"

buffer.WriteString(msg1)
fmt.Print(msg1)

msg2 := fmt.Sprintf(
    "\r\n--%s\r\n"+
        "Content-Transfer-Encoding: base64\r\n"+
        "Content-Disposition: attachment;\r\n"+
        "Content-Type:image/jpg;name=\"test.jpg\"\r\n", boudary)
buffer.WriteString(msg2)
fmt.Print(msg2)

attachmentBytes, err := ioutil.ReadFile("./test.jpg")
if err != nil {
    fmt.Println("ReadFile ./test.jpg Error : " + err.Error())
    return err
}
b := make([]byte, base64.StdEncoding.EncodedLen(len(attachmentBytes)))
base64.StdEncoding.Encode(b, attachmentBytes)
buffer.WriteString("\r\n")
fmt.Print("\r\n")
fmt.Print("图片base64编码")
for i, l := 0, len(b); i < l; i++ {
    buffer.WriteByte(b[i])
    if (i+1)%76 == 0 {
        buffer.WriteString("\r\n")
    }
}

buffer.WriteString("\r\n--" + boudary + "--")
fmt.Print("\r\n--" + boudary + "--")

sendto := strings.Split(to, ";")
err = smtp.SendMail(host, auth, user, sendto, buffer.Bytes())

return err

}
//email_test.go

package email

import "testing"

func TestSendEmailWithAttachment(t *testing.T) {
err := SendEmailWithAttachment("xx@example.com", "passwd", "smtp.xx.com:25", "xx@example.com", "测试附件")
if err != nil {
t.Fatal(err)
}
}
go test 打印输出如下

--THIS_IS_BOUNDARY_JUST_MAKE_YOURS
Content-Type:text/plain;charset=utf-8

这是正文啊

--THIS_IS_BOUNDARY_JUST_MAKE_YOURS
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
Content-Type:image/jpg;name="test.jpg"