How to let mutt always mark mbox as new if it contains new emails?

[Wu, Yue]

Currently, I find that if I enter to a mbox, then quit from it, the mbox's N
mark will be removed, no matter whether there are news mails in it or not, not
what I think preference for me. Can I configure it? I have set the mark_old=no
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Hi,
Wu, Yue