from reading this - I think I see the problem
1.) your sco box relays through your companies mail relay, therefore the DNS lookups are done on that box, not your sco box.
2.) your external mails work well, so that means they are getting to the relay, doing a lookup, finding the host and sending to the host. (all good)
3.) your internal ones are failing, which suggests they are hitting the mail relay, the mail relay is using dns servers that can't see your internal mail domain (either no records or invalid/unroutable MX record) and bouncing it back
3-a.) your mail relay can see your internal dns domains but as it is an "external" box, can't route/get back in to deliver the mail to an internal mail server.
trouble shooting.
get onto the mail relay, do some looksup on your internal domains, check they are valid, then try to reach those machines on port 25.
Solutions. (depending on the output of your trouble shooting above)
a.) configure sco box to not relay for your internal domains
b.) configure your mail relay to use DNS views to it can lookup internal/external hosts correctly.
c.) as suggested cheat the sco box's mail relay by using a host file entry
d.) tell the sco box to send to a specific host, instead of
user@yourdomain.com actually tell it the box that wants the mail,
users@boxname.yourdomain.com
if you're stuck, give us some more output from the trouble shooting.