private void closeServerSocket() {
		try {
			this.serverSocket.close();
			LOGGER.debug("SMTP Server socket shut down");
		} catch (final IOException e) {
			LOGGER.error("Failed to close server socket.", e);
		}
	}
Closes the serverSocket in an orderly way.
src/main/java/de/gessnerfl/fakesmtp/smtp/server/BaseSmtpServerRunnable.java