Exchange email and internet email with outlook 2003

We just upgraded our server to SBS 2003 and also the users to outlook 2003.
each user has outlook set up with exchange email as default, and their
outside email set up as secondary.

both emails are set in the user profile in the user email settings in the
active directory on the server. both are working fine individually, we get
interoffice email and internet email with no problem. if we reply to or send
new interoffice email (using exchange as what is sending the message) and cc
or bcc an internet email, everyone gets their email. however, if we do the
opposite, and reply to or send (using our internet email as what is sending
the email) and cc or bcc an interoffice email (say
(E-Mail Removed)l) those get kicked back as undeliverable
by our outbound internet mail server.

right now my patch is to have the users (all 7 of them) make sure to cc or
bcc eachother's internet email. is there a more appropraiate solution? (one
that doesnt rely on the user's memory to do this?)

with SBS 2000, and outlook XP this was no problem. the settings seem to be
the same, at least where i know to check.

Thanks in advance!
Brad LIttle