ALWAYS send via default SMTP (not Exchange)

Hi everybody!
Server: Exchange 2003 on Win 2003 server standard SP1
Client: Outlook 2003 on Win XP SP2

The emails goes right into the exchange mailboxes with a pop3
connector (SmartPop2Exchange).
From Outlook the new messages goes out trough an external SMTP (that
is configured as default account on Outlook). So far, everything works
fine!
BUT, when I reply to an incoming email (arrived trough the pop3
connector) this messages goes out trough Exchange (that is not
connected to the Internet and can't relay the email) and it will not
be relayed.

So everytime I have to answer to an email, I have to change the
account that will be uset to send it and if I forget it, I will know
only after a couple of hours when exchange send me a message of delay
(and after a couple of days a message of undelivery).

So, is there a way to tell Outlook to use ALWAYS the default SMTP even
for reply/forward?

Thanks,
Andrea