I have anon posting on with email required. I was fiddling around and realized that, if I was logged out, and I posted with my user name (Ipstenu) and the email I used with my registered ID (email@example.com), it would let me post. If I posed with a different user name (jhdgsd) and the email, it wouldn't post.
It occurs to me that having a person be able to 'spoof' registered users is kinda a bad thing.
I saw this post - http://wordpress.org/support/topic/39025?replies=5 - but that's three years old. Is there any way to easily say 'If this email is also used by a registered user, prompt for user to login'?