Email validation JSON endpoint, any error is returned as `code` non-zero value with message else "ok"

email_validator.go

package main

import (
	"flag"
	"fmt"
	"net"
	"net/http"
	"net/smtp"
	"regexp"
	"strings"
	"time"
)

var (
	smptTimeout *time.Duration
	emailRegexp = regexp.MustCompile("^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$")
)

const (
	OK = iota
	SMTP_ERROR
	INVALID_MX_RECORD
	INVALID_EMAIL_FORMAT
)

func response(code int, message string) []byte {
	return []byte(fmt.Sprintf(`{ "message": "%s", "code": %d }`, message, code))
}
func split(email string) (account, host string) {
	i := strings.LastIndexByte(email, '@')
	account = email[:i]
	host = email[i+1:]
	return
}

func dialTimeout(addr string, timeout time.Duration) (*smtp.Client, error) {
	conn, err := net.DialTimeout("tcp", addr, timeout)
	if err != nil {
		return nil, err
	}

	t := time.AfterFunc(timeout, func() { conn.Close() })
	defer t.Stop()

	host, _, _ := net.SplitHostPort(addr)
	return smtp.NewClient(conn, host)
}

func checkEmail(email string) (code int, message string) {

	if !emailRegexp.MatchString(email) {
		return INVALID_EMAIL_FORMAT, "invalid email format"
	}
	_, host := split(email)
	mx, err := net.LookupMX(host)
	if err != nil {
		return INVALID_MX_RECORD, "invalid mx record"
	}
	client, err := dialTimeout(fmt.Sprintf("%s:%d", mx[0].Host, 25), *smptTimeout)
	if err != nil {
		return SMTP_ERROR, "smtp dial error"
	}
	defer client.Close()

	err = client.Hello("example.com")
	if err != nil {
		return SMTP_ERROR, "smtp hello error"
	}
	err = client.Mail("[email protected]")
	if err != nil {
		return SMTP_ERROR, "smtp mail error"
	}
	err = client.Rcpt(email)
	if err != nil {
		return SMTP_ERROR, "smtp reciept error"
	}
	return OK, "ok"
}

func handler(w http.ResponseWriter, req *http.Request) {
	w.Header().Set("Content-Type", "application/json")
	keys, ok := req.URL.Query()["email"]
	if !ok || len(keys[0]) < 1 {
		w.WriteHeader(http.StatusBadRequest)
		w.Write(response(-1, "url param 'email' is missing"))
		return
	}
	w.Write(response(checkEmail(keys[0])))
	return
}

func main() {
	smptTimeout = flag.Duration("timeout", 5*time.Second, "timeout in seconds for smtp")
	port := flag.Int("port", 3001, "server http port")
	flag.Parse()
	http.HandleFunc("/", handler)
	http.ListenAndServe(fmt.Sprintf(":%d", *port), nil)
}