This is a temporary message state. While sending the message to the partner, B2B sets the message state to MSG_WAIT_TRANSMIT. After sending the message, it updates the message state to either MSG_COMPLETE or MSG_ERROR or MSG_WAIT_ACK, depending upon the outcome of sending request. If a message gets stuck in MSG_WAIT_TRANSMIT state then it means there is some problem as B2B could not update it's state. Check the logs to figure out the issue.