docs: @Override
      public void run() {
          try {
              this.keepOn = true;
  
              try {
                  this.serverSocket = this.openServerSocket();
              } catch (final IOException e) {
                  throw new RuntimeException(e);
              }
  
              while (this.keepOn) {
                  try {
                      final Socket clientSocket = this.serverSocket.accept();
                      final POP3Handler pop3Handler = (POP3Handler) this.handler.clone();
                      pop3Handler.setClientSocket(clientSocket);
                      new Thread(pop3Handler).start();
                  } catch (final IOException e) {
                      //e.printStackTrace();
                  }
              }
          } finally {
              this.quit();
          }
      }