Mbox files can be read with Mutt, at least if it's the mbox format
mutt can read. If you have to convert it, this can be done by formail. I haven't found another mbox / maildir reader, but would also be interested.
Maybe KMail can read mbox / maildir also.
formail -ds <sent.mbox >sent-reformatted.mbox
mutt -f sent-reformatted.mbox
formail -ds sh -c 'cat >$FILENO.mail'