Described are a system and method for lossless message delivery between two processing devices. Each device includes a remote direct memory access (RDMA) messaging interface. The RDMA messaging interface at the first device generates one or more messages that are processed by the RDMA messaging interface of the second device. The RDMA messaging interface of the first device outputs a notification to the second device that a message of the one or more messages is available at the first device. A determination is made that the second device has resources to accommodate the message. The second device performs an operation in response to determining that the processing device has the resources to accommodate the message.