All recipients who unsubscribe from your newsletter are placed onto a so-called blacklist. This blacklist ensures that unsubscribed email addresses no longer receive newsletters by mistake. Even if you import your newsletter contacts afresh from an excel file, for example, the original blacklist entries remain on the blacklist.
Furthermore, it is possible to place contacts on to the blacklist straight away when importing. This is useful if you are switching from another newsletter system to Newsletter2Go, for example.