With Hushmail Business you have the following choices when deciding which domain to use:
Your own domain such as yourdomain.com.
A sub-domain of your own domain such as secure.yourdomain.com.
A sub-domain of our hush.com domain such as yourbusiness.hush.com.
If you use a domain of yours for Hushmail Business you will have to register the domain prior to signing up for Hushmail Business.
If you already own the domain you will be required to change the MX records for the domain with the domain registrar. Hush will provide you with the MX record settings at the time of sign up. Changing the MX records for your domain will mean your existing email accounts will no longer receive email, and that email will be sent to your accounts on the Hushmail.com servers.
If you chose to use a sub-domain of your domain then Hush will provide you with the MX record settings at the time of sign up.
If you use a sub-domain of hush.com then Hush will setup the MX records for the domain.