I use a google domain email alias. You have to generate an app password in your gmail account and use the one time generated password as your credentials in the Peplink email settings.
I think GMAIL enforces 2FA now… in any case, once your logged into Google account, manage account > Security > Signing in with Google and go to App Passwords.
Generate the app password using the device and app dropdowns… choose Mail as APP and descriptive device so you know what it is if ever you need to delete.
When you generate, it’ll show a password… which you will use in the Peplink email notification smtp password. You can’t recover this password later, so copy it into the peplink smtp password textbox, or copy it temporarily somewhere.
If you are only emailing to yourself inside your own domain, you can always use the restricted domain aspmx.l.google.com on port 25 with old school SMTP.
OK. Finally got it working (I think). 2FA was turned off for entire domain which was why I was not seeing the option to turn it on at user account level.