Invoking mail(1), you can specify messages such as

    & f :u

(to list unread messages) or

    & p :o

(to print Old messages) or

    & d :r

(to delete messages that have been read), etc.  So they work, but AFAICT, there's no documentation on them in the man-page

    $ man mail | grep ':r'

I imagine the text can be cribbed from [OpenBSD's man-pages for `mail(1)`](https://man.openbsd.org/mail.1#Specifying_messages)