#!/usr/bin/env zsh

setopt EXTENDED_GLOB
res=()
cd $MAILDIR
for mbox (mbox *~mbox(/)) {
    [[ -e $mbox/.inc ]] || continue
    news=($mbox/new/*(N))
    [[ ${#news} != 0 ]] && res=($res $mbox:${#news})
}
print $res