code: func IsExistingEmail(email string) bool {
  
  	if len(email) < 6 || len(email) > 254 {
  		return false
  	}
  	at := strings.LastIndex(email, "@")
  	if at <= 0 || at > len(email)-3 {
  		return false
  	}
  	user := email[:at]
  	host := email[at+1:]
  	if len(user) > 64 {
  		return false
  	}
  	if userDotRegexp.MatchString(user) || !userRegexp.MatchString(user) || !hostRegexp.MatchString(host) {
  		return false
  	}
  	switch host {
  	case "localhost", "example.com":
  		return true
  	}
  	if _, err := net.LookupMX(host); err != nil {
  		if _, err := net.LookupIP(host); err != nil {
  			return false
  		}
  	}
  
  	return true
  }
docs: // IsExistingEmail check if the string is an email of existing domain