Can't register with email address previously used

I *think* this may be a Wordpress issue, but I'm not sure.

I have discovered that once I create an account with a certain email address, I can't use that email address again EVEN if I delete the account that originally used it!

So, for testing, I have created an account with my email address. I then deleted the site and the user. But when I tried to register another site with that email address, I never get the verification email.

Also - is there really NO interface for seeing unverified users??? That seems crazy!

Any guidance?