I have a naive vpn question...

One of our local (family) network 'accounts' moved to another city
and is running behind a cable modem and a linksys BEFSX41 vpn/router.
The cable account only offers pop email and this account would like
to continue to use the imap ssl email account here. All fine and
dandy for retrieving email, but the cable account refuses to relay
email from behind the cable modem.

What would be the easiest and reasonably secure way to let that
box relay through my (postfix) server?

Thanks