Why is my forza not allowing me to download any gift cars or buy stuff off of the storefront or auction house
No idea, but you need to communicate what is happening to Turn 10 via email at email@example.com, with the full details including any other gamertags which have been used on your console.
It will take a while, and you’ll get an automated response initially, because they get a high volume of emails.
what is the POP3 server name?
The email address is firstname.lastname@example.org.
That is all you need.