Kudos for starting up.. These kind of features exist today in many martech systems.
Full disclosure: I am the founder of one : www.appice.io but we are certainly not the first or only ones.
You are right that there are many marketing automation systems for sending emails or push notifications. We are focused on product notifications specific to a user's workflow. The API reflects that and, so does the design of the inbox. For example, the API accepts a category for each notification and you can set preferences per category (one of the categories can be marketing). When adding MagicBell to your frontend, you can write a custom click handler and redirect your user to the specific action they need to take on a notification. A lot of the marketing automation tools also lack an in-app inbox that offers a workflow for these notifications (read/unread/delete).