Although processors become massively multicore and therefore new programming models mix message passing and multi-threading, the effects of threads on communication libraries remain neglected. Designing an efficient modern communication library requires precautions in order to limit the impact of thread-safety mechanisms on performance. In this paper, we present various approaches to building a thread-safe communication library and we study their benefit and impact on performance. We also describe and evaluate techniques used to exploit idle cores to balance the communication library load across multicore machines.