A task was set related to design the message service. Here you should clarify all the necessary information about, be aware the details are important. Try to consider all corner cases, offer solutions, get additional restrictions and adjustments, offer solutions again. Think about scalability (have a look at scale cube it can be helpful), flexibility, and fault-tolerance (reactive programming manifest will be your good helper). If your interviewer asks you to describe ER-model which you offered, please, do it in detail (down to properties and their types).