private void handleExceptionDuringRun(Exception e) {
        LOGGER.error("Unexpected error in the SMTP handler thread", e);
        this.sendResponse("421 4.3.0 Mail system failure, closing transmission channel");
        if (e instanceof RuntimeException re) {
            throw re;
        }
        throw new UnexpectedSMTPServerException("Unexpected exception", e);
    }
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