Select default server to use to send e-mail?

[dude]

I have a client here with a unconventional e-mail server setup. They use
Exchange 2003 for storage and send out e-mails, but each outlook client goes
to a pop3 server out on the internet to pull e-mails in. So each outlook
client has one Exchange account and one POP3 account configured. Exchange is
configured as the default e-mail account. What I noticed recently is that
when someone composes a new e-mail for another user on Exchange, it would use
the POP3 server to send out e-mail. This results in internal e-mails going
out to the POP3 server then coming back in to another POP3 recipient. How do
I change the default outgoing server??

thanks.