private void handleIOExceptionOnRun(IOException e1) {
        if (!this.quitting) {
            // Send a temporary failure back so that the server will try to resend
            // the message later.
            this.sendResponse("421 4.4.0 Problem attempting to execute commands. Please try again later.");
            if (LOGGER.isWarnEnabled()) {
                LOGGER.warn("Exception during SMTP transaction", e1);
            }
        }
    }
The thread for each session runs on this and shuts down when the quitting
member goes true.
src/main/java/de/gessnerfl/fakesmtp/smtp/server/Session.java