'email address already exists'

'email address already exists'

I deleted a user from my organization and want to use the same email address that user had, but the email address seems to still exist somewhere as I get 'email address already exists' when I try and create it.

I have deleted the entire organization and recreated it, so I'm surprised an email address at that domain is still in the system somewhere. If I try and login to that user account I get the message 'this account has been closed due to inactivity'

How do I delete the email address so I can reuse it?