title: Pop3MailReciever is not deleting messages
body: <p>I'm connecting to a POP3 mail server using the Pop3MailReciever in Spring Integration. I want to delete the message after processing it. I tried setting the ShouldDeleteMessages flag, but it does not delete the message. 
  This is the code doing the polling:</p>
  
  <pre><code>@SpringBootApplication
  public class EmailPollerApplication {
  
  public static void main(String[] args) {
      SpringApplication.run(EmailPollerApplication.class, args);
  }
  
  @Bean
  IntegrationFlow pollingFlow() {
      return IntegrationFlows
              .from(mailReceivingMessageSource(), e -&gt; e.poller(Pollers.fixedDelay(60000L)))
              .transform(mailTransformer())
              .transform(requestTransformer())
              .handle(wsGateway())
              .channel("nullChannel")
              .get();
  }
  
  @Bean
  MailReceivingMessageSource mailReceivingMessageSource(){
  
      Pop3MailReceiver pop3MailReceiver = new Pop3MailReceiver("mailserver.example.com", 110, "username", "password");
  
      pop3MailReceiver.setShouldDeleteMessages(true);
      pop3MailReceiver.setMaxFetchSize(1);
  
      MailReceivingMessageSource mailReceivingMessageSource = new MailReceivingMessageSource(pop3MailReceiver);
      return mailReceivingMessageSource;
  }
  </code></pre>
  
  <p>This is the code from Pop3MailReciever that is supposed to be deleting the message:</p>
  
  <pre><code>@Override
  protected void deleteMessages(Message[] messages) throws     MessagingException {
      super.deleteMessages(messages);
      // expunge deleted mails, and make sure we've retrieved them before closing the folder
      for (int i = 0; i &lt; messages.length; i++) {
          new MimeMessage((MimeMessage) messages[i]);
      }
  }
  </code></pre>
  
  <p>The super.deleteMessages(message) is setting the DELETED flag on the messages. This is all well and good, but nothing is happening on the server. I had tcpdump going while my app was running and the DELE POP3 command was never run.</p>