I’m using the Newsletter plugin successfully, there is only one problem: The return path I configured is not used. Instead, bounces are sent to a default address generated by the server. The same happens if the return path field is left blank. After reading your documentation regarding the return path, I contacted my provider. They say that the problem must be within the plugin and that it uses the PHP mail() function incorrectly and the return path is not properly specified under “additional_parameters”.
I would be surprised if there was such a significant bug in such a widely used plugin, but could you check this and/or give me a statement I can tell my provider that (and why) the plugin cannot be the reason?