The emqtt server is licensed under the MPL. 

The files below copied from rabbitmq and licensed under LICENSE-MPL-RabbitMQ:

credit_flow.erl
file_handle_cache.erl
gen_server2.erl
priority_queue.erl
supervisor2.erl
tcp_acceptor.erl
tcp_acceptor_sup.erl
tcp_listener.erl
tcp_listener_sup.erl

If you have any questions regarding licensing, please contact ery.lee@gmail.com