在调用以下我的代码片段时：

Message message_in = null;

inbox instanceof IMAPFolder

IMAPFolder f = (IMAPFolder)inbox;

f.idle();

System.out.println("IDLE done");

message_in = inbox.getMessage(inbox.getMessageCount());

message_in.setFlag(Flags.Flag.DELETED, true);

inbox.expunge();

我收到错误消息：

javax.mail.MessagingException: A9 NO Command received in Invalid state.;

nested exception is:

com.sun.mail.iap.CommandFailedException: A9 NO Command received in Invalid state.

at com.sun.mail.imap.IMAPMessage.setFlags(IMAPMessage.java:905)

at javax.mail.Message.setFlag(Message.java:578)

at com.name.vmmonitor.main.VMMonitor.startPolling(VMMonitor.java:160)

at com.name.vmmonitor.main.VMMonitor.main(VMMonitor.java:283)

Caused by: com.sun.mail.iap.CommandFailedException: A9 NO Command received in Invalid state.

at com.sun.mail.iap.Protocol.handleResult(Protocol.java:351)

at com.sun.mail.imap.protocol.IMAPProtocol.storeFlags(IMAPProtocol.java:1589)

at com.sun.mail.imap.protocol.IMAPProtocol.storeFlags(IMAPProtocol.java:1574)

at com.sun.mail.imap.IMAPMessage.setFlags(IMAPMessage.java:901)

... 3 more

有什么问题？我可以调用诸如message_in.getSubject()之类的方法,但为什么不调用message_in.setFlag()？