Shell accounts are generally a pain in the butt to deal with, but you
have two options:
1) If they want a mail only account, you can give them a PPP/mail
account, but put filters into the PM to not allow them access outside
of your network. This in general works pretty good and you can add
your own touch to make it useable.
2) Setup a FreeBSD/Linux box to handle the shell accounts.
I prefer 1, since its easier to maintain. :)
-- Dale E. Reed Jr. Internet Engineering Associates Providing Internet Solutions for Today