I am not an expert at this but I have my Outlook client setup to do both - connect to internal mail server when at work and connect to my ISP for external mail outside.

In the client select Tools ->Services and install Internet mail. Configure the ISP mail server etc
I cannot remember how to switch between the two, but you can compose a new e-mail and send it to either internal or external.

Anything you reply to from the external ISP server will be sent back via the external server.

Maybe some one else can enlighten you more

Golam