We have a large up to date list of fake/temporary email sites and registering from these is impossible. A list of domains (@whatever.com) used to sign up on the site is also automatically made and this is browsed through to check the sites are all legit now and then too.
Also, if you are using a free email account like gmail, hotmail, googlemail, yahoo, etc etc etc you will not be able to register here either.
This is to prevent things like people having multiple accounts and general abuse
Please just use your internet account that came with your ISP or if you're at university or whatever the one there. Just anything that isn't free, so that we can be fairly sure accounts are unique.