zabbix smtp send email

zabbix_smtp.go

package main

import (
	"encoding/base64"
	"fmt"
	"net/mail"
	"net/smtp"
	"os"
	"strings"
)

func encodeRFC2047(String string) string {
	// use mail's rfc2047 to encode any string
	addr := mail.Address{String, ""}
	return strings.Trim(addr.String(), " <>")
}

func main() {
	from := mail.Address{"监控邮件", "[email protected]"}
	to := mail.Address{"收件人", os.Args[1]}

	subject := os.Args[2]
	body := os.Args[3]

	header := make(map[string]string)
	header["From"] = from.String()
	header["To"] = to.String()
	header["Subject"] = encodeRFC2047(subject)
	header["MIME-Version"] = "1.0"
	header["Content-Type"] = "text/plain; charset=\"utf-8\""
	header["Content-Transfer-Encoding"] = "base64"

	message := ""
	for k, v := range header {
		message += fmt.Sprintf("%s: %s\r\n", k, v)
	}
	message += "\r\n" + base64.StdEncoding.EncodeToString([]byte(body))

	addr := "smtp.mst365.cn:25"
	auth := smtp.PlainAuth("", "[email protected]", "456", "smtp.mst365.cn")

	err := smtp.SendMail(addr, auth, from.Address, []string{to.Address}, []byte(message))
	if err != nil {
		panic(err)
	}
}

I tried that but getting bellow error:
panic: x509: certificate has expired or is not yet valid

goroutine 1 [running]:
main.main()
/home/mujibur/zabbix_smtp.go:44 +0x9b4

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
/usr/local/go/src/runtime/asm_amd64.s:2232 +0x1
exit status 2

Thank you!