Duplicate emails - Spring Integration Mail

Question

I am trying to setup a integeration flow to read UNSEEN emails from a mailbox and do some process on those. I have done the initial setup as below and I am having two issues.

   @Bean
   public IntegrationFlow mailListener() {
      return IntegrationFlows.from(Mail.imapInboundAdapter(CONNECTION_URL)
                              .shouldDeleteMessages(false)
                              .shouldMarkMessagesAsRead(true)
                              .searchTermStrategy(this::searchTerm)
                              .simpleContent(true)
                              .autoCloseFolder(false),
                      e -> e.poller(Pollers.fixedRate(5000).maxMessagesPerPoll(100).taskExecutor(taskExecutor())))
              .<Message>handle((payload, header) -> logMail(payload))
              .get();
   }

   @Bean
   private TaskExecutor taskExecutor() {
      ThreadPoolTaskExecutor threadPoolTaskExecutor = new ThreadPoolTaskExecutor();
      threadPoolTaskExecutor.setCorePoolSize(10);
      threadPoolTaskExecutor.setMaxPoolSize(20);
      threadPoolTaskExecutor.setQueueCapacity(200);
      threadPoolTaskExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
      threadPoolTaskExecutor.setKeepAliveSeconds(500);
      threadPoolTaskExecutor.setWaitForTasksToCompleteOnShutdown(true);
      threadPoolTaskExecutor.initialize();
      return threadPoolTaskExecutor;
   }

   private SearchTerm searchTerm(Flags flags, Folder folder) {
      SearchTerm[] searchTerms = {new FlagTerm(new Flags(Flags.Flag.SEEN), false)};
      return new AndTerm(searchTerms);
   }

Issue 1 I am seeing a lot duplicate messages. The same emails are read again. I have also added a SearchTerm to filter the messages. Am I doing something wrong here?

Issue 2

When there are a lot of emails in mailbox (approx: 600000). ImapInboundAdapter tries to fetch all of them at once even when I have set max-messages-per-poll on the Poller. If I set max-fetch-size on the adapter then, It only fetches the configured number of messages but then processing is really slow around 5-10 sec/email. Fetch process ( Search -> flagging all the emails --> fetching the content ) takes a lot of time . So my second question here is , How can I increase the performance to process around 200000 emails on daily basis?

Any Suggestions would be helpful. Thanks