Adding my domain works but my address has an "address already exists" error

Adding my domain works but my address has an "address already exists" error

Hi, I've just added one of my domains (flaxx.net) as a test before paying for the Zoho service. This has worked fine, but when I add my address ( simon@flaxx.net) Zoho Mail tells me that the address already exists. How can this be? I don't think I've done anything to configure that yet.