tl;dr: Is there a way to find the email address required by the factory reset protection?
This is what happened: Mother in law gets a new phone (first smartphone) for Christmas, and to simplify things the giver set up the phone pre-christmas. Tries to set it up with FirstnameLastname@gmail.com, that address is already taken, so they just picked one of the suggestions of FirstnameLastnameemail@example.com; Something didn't work as they wanted, factory reset the phone according to random internet instructions, but forgot to write down the exact email address.
Now the phone hit the factory reset protection and requires the exact email address to continue. We know the password to the account, but don't know ~4 characters of the email address.
I've found instructions to circumvent the FRP by downloading APKs onto the phone, but prefer to avoid that if possible.