mark for unread messages in a mailbox?

[Qingjia Zhu]

  Dear mutt users,

        I have set mark_old=no in my .muttrc, and folder_format
        is by default. If I opened a mailbox and leave some unread
        messages still unread there, I want mutt to keep a 'N' mark
        for that mailbox the next time I start mutt, but not just
        clear this mark, you know I have bad memory. How to workaround
        this? thanks.

  Cheers,