Takes, as input, an e-mail address and returns the first user name from the data source where the e-mail address matches the supplied email parameter value.

If no user name is found with a matching e-mail address, null is returned.

If multiple user names are found that match a particular e-mail address, only the first user name found should be returned. You can implement a custom mechanism for ensuring a unique e-mail address for each user name such as the SqlMembershipProvider.RequiresUniqueEmail property supported by the System.Web.Security.SqlMembershipProvider provider.