hierarchical folder in mutt?

[Jun Sun <jsun@xxxxxxxxxx>]

I like to group a bunch of open source mailing lists under a single
(sub-)folder so that the top-level mboxes are not too crowded.

Is this possible?

I searched web and seem to get conflicting information.  And in any case
did get any specific instructions.

I tried to list a directory as a "mailbox".  The directory shows up
fine.  However, it is not quite "mail folder" in the sense that
mutt does not indicate whether any new mails exists under that folder,
nor does mutt know the files under that folder are mailboxes.

Thanks in advance.

Jun