public class ValidateUserNotificationArgs : Dynamicweb.Extensibility.NotificationArgs 

[Dynamicweb.Extensibility.Subscribe(Dynamicweb.Modules.UserManagement.Notifications.UserValidated)]
public class ValidateUserObserver1 : Dynamicweb.Extensibility.NotificationSubscriber
{
    public override void OnNotify(string notification, Dynamicweb.Extensibility.NotificationArgs args)
    {
        if (args == null || !(args is Dynamicweb.Modules.UserManagement.ValidateUserNotificationArgs))
            return;
        Dynamicweb.Modules.UserManagement.ValidateUserNotificationArgs item = (Dynamicweb.Modules.UserManagement.ValidateUserNotificationArgs)args;
        Dynamicweb.Modules.UserManagement.User user = item.Subject;
        Dictionary<string, Dynamicweb.Modules.UserManagement.UMFormField> error = item.Errors;
        if(!Dynamicweb.Base.ValidateEmail(user.Email))
        {
            string key = string.Format("UserManagement_Form_{0}", "Email2");
            error.Add(key, new Dynamicweb.Modules.UserManagement.UMFormField(key, "Email", "The email is not valid from. Custom! "));
        }
    }
}