Active Directory username as email address ?

Does Microsoft support an email address as a domain user name in Active Directory where the domain of the email address is partners domain? Are we better off storing email addresses as usernames in ADAM or should we create our own Membership database? For example, "MyDomain\UserName" as "MyDomain\yourUserName@YourDomain.com". What is the best practice for this scenario?

Another way to look at is is by seeing the fully qualified LDAP address. The address in the scenario above would then become[CN=myEmailUserName@MyDomain.com,OU=MyOU,DC=MYSubDC,DC=MyDC,DC=com]. Please let me know if that is best practice with the ActiveDirectoryMembershipProvider.