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
Now as you know, has different rules for so you will need to do the following to make it work:
1. Make sure the membership provider treats email as a unique attribute for users so you don’t end up with more than one user attached to the same email:
<add name="sql" type="System.Web.Security.SqlMembershipProvider" connectionStringName="core" applicationName="sitecore" minRequiredPasswordLength="1" minRequiredNonalphanumericCharacters="0" requiresQuestionAndAnswer="false" requiresUniqueEmail="true"security
Put email into both username and email properties (fields) of a user during the registration (can be handled via code).
Introduce the following entry into the web.config’s <settings> section. The “value” attribute parameter contains the regular expression used in the Create User dialog within User Manager. This regex should allow emails, otherwise Sitecore will fallback on a default regex that does not allow it.
<setting name="AccountNameValidation" value=".+" />
If you want to handle the case when email needs to be changed, either provide an extranet form for the profile section on your website or you can even take it one step forward – modify the EditUser dialog within User Manager to have this ability.