[Postfix] Dial-Up Dilemma

[Advo]

I have a small network at home, 4 workstations and one server. The server is connected to the internet via adsl. The clients are not allowed to have direct connections to the internet, everything is proxied (no masquerading or such).
But there's a (small) problem with the email system. I 've set up Postfix etc. according to the guide Email System For The Home Network and added a antivirus-scanner. Since I'm on dial-up, I have to use a relayhost for mail delivering. The problem is that every user has it's own external mail account. So I can't send mail from usera and userb through the same relayhost. I read about enabling "sender_based_routing" in postfix >2.0, but it didn't work for me (in fact, it looks like this option does not really exist). Luckily all external services support smtp-after-pop, so I set "defer_transports = smtp" and run a cronjob every hour for sending mails just a few minutes after fetchmail. But this has some drawbacks - first that it is far from being safe since I never know how many mails an account contains (I have to fetch mail from 12 mailboxes) and therefore the timing for delivering could be bad. Secondly every mail needs about 2 hours to leave the server. This is because the antivirus-scanner is invoked as a content filter via smtp in the master.cf.

Is there a possibility to made things a little more secure? I had a quick look at exim which has the ability to use more than one smarthost. But I'm not sure I understood how things work there.

~ Advo