It does not matter if your domain was  registered by HotDoodle or transfered to HotDoodle, any domain can be used if it directs email traffic to the HotDoodle email servers.

Email traffic is directed per the domain's MX records. (Read about domain traffic direction.) If the domain uses HotDoodle nameservers the MX records are automatically configured. If the domain uses nameservers from somewhere else they will have to be set to direct email traffic to the HotDoodle email servers.