checkmail.go

package checkmail

import (
	"errors"
	"fmt"
	"net"
	"net/smtp"
	"regexp"
	"strings"
	"time"
)

type SmtpError struct {
	Err error
}

func (e SmtpError) Error() string {
	return e.Err.Error()
}

func (e SmtpError) Code() string {
	return e.Err.Error()[0:3]
}

func NewSmtpError(err error) SmtpError {
	return SmtpError{
		Err: err,
	}
}

const forceDisconnectAfter = time.Second * 5

var (
	ErrBadFormat        = errors.New("invalid format")
	ErrUnresolvableHost = errors.New("unresolvable host")

	emailRegexp = regexp.MustCompile(`(?m)^(((((((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?)|(((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?"((\s? +)?(([!#-[\]-~])|(\\([ -~]|\s))))*(\s? +)?"))?)?(((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?<(((((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?(([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+(\.([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+)*)((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?)|(((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?"((\s? +)?(([!#-[\]-~])|(\\([ -~]|\s))))*(\s? +)?"))@((((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?(([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+(\.([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+)*)((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?)|(((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?\[((\s? +)?([!-Z^-~]))*(\s? +)?\]((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?)))>((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?))|(((((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?(([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+(\.([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+)*)((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?)|(((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?"((\s? +)?(([!#-[\]-~])|(\\([ -~]|\s))))*(\s? +)?"))@((((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?(([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+(\.([A-Za-z0-9!#-'*+\/=?^_\x60{|}~-])+)*)((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?)|(((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?\[((\s? +)?([!-Z^-~]))*(\s? +)?\]((((\s? +)?(\(((\s? +)?(([!-'*-[\]-~]*)|(\\([ -~]|\s))))*(\s? +)?\)))(\s? +)?)|(\s? +))?))))$`)
)

func ValidateFormat(email string) error {
	if !emailRegexp.MatchString(strings.ToLower(email)) {
		return ErrBadFormat
	}
	return nil
}

// ValidateMX validate if MX record exists for a domain.
func ValidateMX(email string) error {
	_, host := split(email)
	if _, err := net.LookupMX(host); err != nil {
		return ErrUnresolvableHost
	}

	return nil
}

// ValidateHost validate mail host.
func ValidateHost(email string) error {
	_, host := split(email)
	mx, err := net.LookupMX(host)
	if err != nil {
		return ErrUnresolvableHost
	}
	client, err := DialTimeout(fmt.Sprintf("%s:%d", mx[0].Host, 25), forceDisconnectAfter)
	if err != nil {
		return NewSmtpError(err)
	}
	client.Close()
	return nil
}

// ValidateHostAndUser validate mail host and user.
// If host is valid, requires valid SMTP [1] serverHostName and serverMailAddress to reverse validation
// for prevent SPAN and BOTS.
// [1] https://mxtoolbox.com/SuperTool.aspx
func ValidateHostAndUser(serverHostName, serverMailAddress, email string) error {
	_, host := split(email)
	mx, err := net.LookupMX(host)
	if err != nil {
		return ErrUnresolvableHost
	}
	client, err := DialTimeout(fmt.Sprintf("%s:%d", mx[0].Host, 25), forceDisconnectAfter)
	if err != nil {
		return NewSmtpError(err)
	}
	defer client.Close()

	err = client.Hello(serverHostName)
	if err != nil {
		return NewSmtpError(err)
	}
	err = client.Mail(serverMailAddress)
	if err != nil {
		return NewSmtpError(err)
	}
	err = client.Rcpt(email)
	if err != nil {
		return NewSmtpError(err)
	}
	return nil
}

// DialTimeout returns a new Client connected to an SMTP server at addr.
// The addr must include a port, as in "mail.example.com:smtp".
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 split(email string) (account, host string) {
	i := strings.LastIndexByte(email, '@')
	// If no @ present, not a valid email.
	if i < 0 {
		return
	}
	account = email[:i]
	host = email[i+1:]
	return
}