SMTP issue

I've just setup a qmail server using SME 5.

Although we currently use it in our LAN, we plan to set it up at our co-location site along with our other servers.

The issue is that we, along with other parties who we'll setup email for, will no longer become part of the server's LAN, therefore we have the problem where we cannot send mail out using our mail server. So far, I know that 'obtuse-smtpd' will not send email from external sources to other external networks. I know that using the ISP's SMTP would work, but I would much rather find a way if possible to use our server to selectively relay outgoing mail without completely opening our ports.

If there is a way other than using the PPTP VPN method, I would greatly appreciate any response.

Thanks,

Sang Chang