You can still buy your own domain and get as many new email addresses which pass your test as you want. So you probably need another solution to your problem.
What I noticed what several (large) services these days do, is ask for your credit card number on signup (even if they promise not to bill you).
You could. But most people won't. The ones that do this will game your system regardless; they have the time and resources to. It's about making it not worth it to buy a domain and set up MX forwarding/gmail federation or whatever, just for $5.
Many email systems have an idea of a catchall account. I can have a domain for which cestith@mydomain goes into one account and anything else @mydomain goes into a second account.
What I noticed what several (large) services these days do, is ask for your credit card number on signup (even if they promise not to bill you).