I want to register my domain "flextodo.com" to your business mail service but I am getting an error when I want to register my domain.

The error  "* This domain is already associated with this account "a*****n@f*****". Sign in" but does not have any account which is associated with this domain. I think this domain is already registered by the different person but now I buy this domain from GoDaddy. So please help me for creating an account for consuming your service.