A circuit arrangement, method, and program product for communicating data between hardware threads of a network on a chip processing unit utilizes shared inboxes to communicate data to pools of hardware threads. The associated hardware in the pools threads receive data packets from the shared inboxes in response to issuing work requests to an associated shared inbox. Data packets include a source identifier corresponding to a hardware thread from which the data packet was generated, and the shared inboxes may manage data packet distribution to associated hardware threads based on the source identifier of each data packet. A shared inbox may also manage workload distribution and uneven workload lengths by communicating data packets to hardware threads associated with the shared inbox in response to receiving work requests from associated hardware threads.