An Internet of Things (IoT) platform of a new horizontal type is presented in this paper, which was developed for fast IoT system prototyping and speedy roll-out of respective services and applications. Some design aspects of the platform, named EMULSION, are presented w.r.t. elaborated open-source software components. The platform supports the supply of a variety of IoT services to users, delivered via heterogeneous access communication networks through a variety of user electronic devices, in a context-aware fashion ensuring that the best Quality of Experience (QoE) is delivered to each user anytime-anywhere-anyhow, in line with the Always Best Connected and best Served (ABC&S) communication paradigm. More specifically, the paper is focused on presenting some test aspects of a distributed message broker, developed as a lightweight middleware solution for EMULSION, providing high reliability and high concurrency of operations.