Hi,
empty subject is one of attributes which avast! uses during heuristic analysis of email messages. It "can be" an indication of virus. If you don't want use testing for empty subject, you can disable it in avast! settings. BUT! After almost one year experience with this feature, we decided to exclude testing for empty subject from all defined sensitivity (Normal, Medium, High). It can be accesible only in Custom settings. The reason is that it brings more "damages" than benefits. This manner will be available in next program update.
pavels