Okay, the short answer is "yes, you are". The long answer is that the system will only decline e-mails that are duplicates of ones linked to existing accounts. If your e-mail is being automatically rejected, then someone has used it to register before.
You can either feed it into the Lost Password feature, and see what comes out, or you can get yourself another e-mail, and register again. Please note that if your IP matches any others used to register for, or play the game, then your application will be declined for that reason instead.
____________"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Only fools and children dream of heroes.