If you're talking about
https://www.guerrillamail.com/ , it may be that they have blocked many temporary email services for a couple of reasons that I can think of:
Email is supposed to be the lifeline of account related issues/updates. Such as, if Cryptic/PW were hacked and information stolen, you would be notified via account email.
To decrease exploitation of F2P unlimited account registration per physical user for monetary gains in-game.
That's speculation really....I don't know for sure.
EDIT: If you have used a gmail address at any point, every Gmail address comes with an alternate @googlemail.com and all mail sent to that alternate will go to the original @gmail.com inbox. You can also use "+" or "." (periods) anywhere in the gmail address to register as a different address in Neverwinter BUT gmail ignores this on thier exchange and sends it right to the one without the + or .
Source:
Official Gmail Blog: 2 hidden ways to get more from your Gmail address