email address has already been used during signup

After I installed wordpress mu (2.8.5.2) I created several testing accounts for myself to see if some default settings for new blogs are working the way they should. I have a limited amount of email adresses, so after a while I delete all the blogs and users from the admin panel. So the email address shouldn't be in use anymore.

But when I try to create a new blog with a email address that I used before it still comes up with this error

"That email address has already been used. Please check your inbox for an activation email. It will become available in a couple of days if you do nothing."

But the email address that I used to create a new blog is no where to be found in the admin panel. I had a look in the database, and I don't see it there either.

Is there any way to reset the used email address? Or is this some kind of bug?