<http://schema.org/description>
"It is pretty common to have email served as username for authentication in web systems for both visitors and internal users. Sitecore Content Management System With 5.3 Now as you know, has different rules for so you will need..."^^<http://www.w3.org/2001/XMLSchema#string>
<http://sitecoreblog.alexshyba.com/sitecore_6_quick_tip_email_as_user_name_for_authentication/>