Forgotten e-mail associated with an account

This does suggest Microsoft account is associated with the wrong email. What I would do is on the Xbox with the Gamertag you believe is the right one is check the E-mail address associated with it. A Gamertag is associated with a Microsoft Account but
the E-mail address that is associated with a Microsoft Account\E-mail address can change. Hope that helps and makes sense.

I've forgotten the E-mail that I used to create the account, and when I normally try to log in to the account on my Xbox 360 it says that it needs the E-mail and the password.

Unfortunately, it's not as simple as just changing the e-mail; you need to go through the steps that Mister Quimby has linked for you if you want to keep both so that you can associate e-mail A (which is tied to account A) with Account B (which has e-mail
B associated with it). Because A is tied to an account, it cannot be used for another account.

If you don't want to keep both, you could try
closing/canceling the account associated with the e-mail that you want to use. Then the e-mail would no longer be associated with an account and should be available to you. Please note that you'll lose any licenses/content access associated with that account
if you close it.

You check your e-mail associated with the account? They send an e-mail with a brief explanation when an account is suspended.

Change the e-mail associated with the silver account ... (to something new ,NOT the e-mail for your main account) then apply the 2 day code.