Is there a way to set Outlook 2k3 to use an Exchange server for one email
address and an external(my ISP) for another email address?

I can receive but not send. Here is what I get if I try to send and email
from the external account

Your message did not reach some or all of the intended recipients.

Subject: test
Sent: 6/20/2006 3:13 PM

The following recipient(s) could not be reached:

Lname, Jay on 6/20/2006 3:13 PM
553 sorry, that domain isn't in my list of allowed rcpthosts
(#5.7.1)