void Application_Start(object sender, EventArgs e) 
{
    System.Web.ApplicationServices.RoleService.SelectingProvider += 
        new EventHandler<System.Web.ApplicationServices.SelectingProviderEventArgs>(RoleService_SelectingProvider);
}

void RoleService_SelectingProvider
    (object sender, System.Web.ApplicationServices.SelectingProviderEventArgs e)
{
    if (e.User.Identity.Name.IndexOf("@example.com") > 0)
    {
        e.ProviderName = "EmployeeRoleProvider";
    }
    else
    {
        e.ProviderName = "CustomerRoleProvider";
    }
}