i think faces expects upas/fs to be available on /n/mymail/mbox. /n/mymail presents a directory of mailboxes.
typically one would start upas/fs on /mail/box/$user/mbox and then add imap mboxes by ; echo open /imap/mail.mac.com mymail>>/mail/fs/ctl & ; faces -m/mail/box/mymbox unfortunately, ned doesn't like to read mailboxes created this way. i'll put my version of ned up on sources. i don't know about acmemail. - erik