Hi--
If this helps anyone, I don't remember how I got to this but this is what worked for me after researching all over internet:
Go to your e-mail settings on Express Invoice use this: smtp.gmail.com:587 instead of smtp.gmail.com
Next go to your google account and under security on the Signing in to Google section go to App password, it will prompt you for your gmail account password. Once your in select app click other type in Express Invoice, select your device and click generate, copy the long password.
After that go back to e-mail setting on Express Invoice and paste that long password that you just generated onto the password section there.
Hope this makes sense.