Xbox email account

So recently the Xbox One asked me to put in the password to the account again, but since its my dads account I dont know it. I tried many different things not knowing that after so many it would lock me out. Now that i cant look on the Xbox itself to find the email ive resorted to the internet. Ive tried signing in online with the email my dad has told me but it seems he forgot what email he used considering every email ive tried isnt associated with an account at all. I believe the password is right, just not the email he used. What can I do to figure out the email connected to the account? I've googled like crazy and everything at least requires the email which i do not have.