Issue with: Add a Domain

I'm trying to set up free email using Custom Domains.

I set up the wrong email address for the Super Admin on my first attempt to configure this service.  I figured the easiest thing to do, was to delete everything and start over. 

 

Except now I get the error:  Domain already exists.

How do I get the domain deleted, so I can start over from scratch?

Please assist & thank you,

Tim