send/receive options not in effect

My OL 2003 client is connecting to an Exchange server and a pop server. I
have modified the default send/receive group to receive mail from both
servers but to send mail only through the pop server to avoid issues with
the very small message size restictions on the Exchange server, which I
don't manage. For some reason though, whenever I reply to a message
received through the Exchange server it still wants to send it through the
Exchange server. Just above the "To" fiels a message box appears theat says
"This message will be sent via microsoft Exchange server". Is there any way
to force all messages to be sent throu exchange? TIA.