Recently, I was tasked to work on separating 2 companies in Postini that were no longer under the same organization. In order to create the new account, I had to first delete the domain under the old Postini config which also required deleting all the users. I ran into a slight issue with this.
In order to delete the Org, you must delete all the domains under the Org.
In order to delete the domain, you must delete all the users under the Org.
You are not allowed to delete firstname.lastname@example.org because the Org must have a default user.
You cannot create a new account for domain.com because the domain is in use under another Postini account.
The trick here is to assign a new default user for the org and then delete everything. Simply go to the Org General Settings and scroll down until you see Default User.
From here, you can change that to any user that already exists under your account. Even one under a different domain and Org. While this was a very simple solution, it was a little hard to realize what needed to be done because of the steps I went through to attempt to simply delete an Org from a Postini account.