Hi KvaGram
Umbraco doesn't offer this functionality out of the box, so this is most likely something custom that has been build on top of your Umbraco installation. There is also a few third party packages that provide newsletter functionality.
So without more information, it's hard to come with any pointers, but if you can share som more details about how you'd interact with the newsletter, it might help spot something.
Eg. if you subscribe to the newsletter, do your get a confirmation mail? If you're using a third party vender for handling the newsletter, maybe there is something in the header or footer of the email than mentions said vendor?