I tried entering the following email address and received a 'Error: Email Address contains invalid characters' validation error when registering a new user:
hooknc!@hotmail.com

According to the wiki article on email address' the following special characters are allowed:
! # $ % & ' * + - / = ? ^ _ ` { | } ~

Please see the following page for more information:
http://en.wikipedia.org/wiki/E-mail_address