There are four alternatives to using the built-in PIN email system:
Use GMail to send the PIN email. Read more about this here.
For Google Mail users, you can use Google's authentication process, "Sign in with Google".
Implement Single Sign-On. This requires an Enterprise plan.
Send the PIN via Twilio.
You can read more about these options here.
If your users are having trouble finding their PIN email, check out this article.