Sorry for the late reply.
To remove/alter the UI element, I can just edit the elastic html template if I am not mistaken.
However, I searched for a plugin with the functionality to add a recipient to every message, but did not find one. Therefore, I am assuming I have to write a small plugin myself.
I already found the Plugin API description on GitHub, however is there a small plugin that does something similar, like adding a bcc or something like that, which I could use as template?