We've created an account for our website mail server several months ago. Now when we want to manage our accounts because we forgot the registered email address, we are not able to login to control panel. 

Also when we want to sign up again, system says this domain is already registered. 

We have access to both our domain control panel and host panel. 

Could you please help to remove this domain or maybe find the email address that has access to control panel.

Thank you