Accessing my pop3 & smtp account from internal exchange account

My organisation has pop3 accounts with an external host with an ip address say, x. We have an internal exchange server 2003 with a private address in 192.y. Our organisation is connected to internet via a pc on which we have enabled internet sharing. It gets a public ip address from isp for dsl adapter and 192.168.0.1 (for internal lan adapter.). I would like to send & receive my mail on my external host from my exchange server 2003 leaving a copy of the mails on the external host. I would also like to send receive from my internal users to the external accounts. Please help.