问Go Golang SMTP脚本。需要添加密件抄送表头

EN

package main

import (
   "fmt"
   "log"
   "net"
   "net/mail"
   "net/smtp"
   "crypto/tls"
)

func main() {

from := mail.Address{"", "username@example.tld"}
to   := mail.Address{"", "username@anotherexample.tld"}
subj := "This is the email subject"
body := "This is an example body.\n With two lines."

headers := make(map[string]string)
headers["From"] = from.String()
headers["To"] = to.String()
headers["Subject"] = subj

message := ""
for k,v := range headers {
    message += fmt.Sprintf("%s: %s\r\n", k, v)
}
message += "\r\n" + body

servername := "smtp.example.tld:465"

host, _, _ := net.SplitHostPort(servername)

auth := smtp.PlainAuth("","username@example.tld", "password", host)

tlsconfig := &tls.Config {
    InsecureSkipVerify: true,
    ServerName: host,
}

conn, err := tls.Dial("tcp", servername, tlsconfig)
if err != nil {
    log.Panic(err)
}
c, err := smtp.NewClient(conn, host)
if err != nil {
    log.Panic(err)
}
if err = c.Auth(auth); err != nil {
    log.Panic(err)
}
if err = c.Mail(from.Address); err != nil {
    log.Panic(err)
}
if err = c.Rcpt(to.Address); err != nil {
    log.Panic(err)
}
w, err := c.Data()
if err != nil {
    log.Panic(err)
}
_, err = w.Write([]byte(message))
if err != nil {
    log.Panic(err)
}
err = w.Close()
if err != nil {
    log.Panic(err)
}
c.Quit()
}